(written from a Production point of view)
Redshirt is a term used by fans and staff of Star Trek to refer partially to the characters who wear red Starfleet uniforms, and mainly to refer to those characters who are expendable, and quite often killed, sometimes in great numbers.
In the era regularly depicted in Star Trek: The Original Series, red uniforms were worn by members of the operations division. They normally performed security, engineering or support services (such as communications officers, administrators and yeomen) aboard starships and starbases.
Of these, the security personnel were quite expendable. TOS: "The Changeling" and "The Apple", in TOS Season 2, both featured four security redshirts dying in each episode. "The Changeling" has the most anonymity involved; all but one of the redshirts that die are unnamed, the other being Carlisle (Nomad also "killed" Mr. Scott, but was kind enough to restore him at Kirk's request).
In "Obsession", the dikironium cloud creature kills three security guards that are shown, all in red shirts, including Ensign Rizzo. One redshirt, however, is lucky enough to be transported to the Enterprise in critical condition. (The creature also kills one crewman aboard the ship, but the precise color of his shirt is never shown.) One of the vampire cloud's victims doesn't quite count – Mr. Leslie would have been a fourth redshirt killed in the outing, but a mention of him surviving was cut from the episode's final edit. He clearly appears in later episodes, so it's probable he either has a twin or survived the attack.
TOS: "Where No Man Has Gone Before" is the first to feature a redshirt and has the most associated deaths; twelve crew people were lost, nine of whom died instantly at the galactic barrier, and three more of the twelve victims perished in events at Delta Vega. We saw only the latter three die on screen, but we know that none of them were technically redshirts, as there were no red uniforms of the design they used in that episode, reused from TOS: "The Cage" (which, itself, featured three off-screen deaths). The operations division was wearing beige at this point.
None of the officers was really killed in Star Trek: The Animated Series, but Star Trek: The Next Generation introduced a new twist to the "redshirt" lore, as the uniform colors switched and operations division wore the gold uniforms while the command division took on the red shirts. They also became likely to die; a theme of crew deaths was dominated by the continuous loss of their flight controller. Lieutenant Torres probably survived TNG: "Encounter at Farpoint", but the TNG era lost Haskell, Monroe, Dern, Nell Chilton, Hawk and Branson.
Non-Enterprise crew redshirts hardly fared any better, demonstrating an alarming propensity for being killed, possessed and/or otherwise coming to bad ends. Notable examples included Captain Tryla Scott, Commander Dexter Remmick, and the entire Senior Admiralty at Starfleet Command, who were taken over by alien parasites in "Conspiracy". Admiral Mark Jameson was killed by a de-aging medicine overdose in "Too Short a Season", Admiral Erik Pressman was arrested in disgrace for violating the Treaty of Algeron in "The Pegasus", and Admiral Matthew Dougherty was murdered by his Son'a co-conspirators in Star Trek: Insurrection.
The only TNG episodes to feature death in large numbers had to do with the Borg. TNG: "Q Who", "The Best of Both Worlds" and "The Best of Both Worlds, Part II" noted eighteen off-screen deaths (although the latter probably totaled a few more in later scenes).
There was still danger in the Security division in TNG, despite their change to the gold color. Natasha Yar, the first Chief of Security of the USS Enterprise-D, was killed by Armus very early in the series (namely, in TNG: "Skin of Evil").
The Star Trek films kept the crew losses low for the most part, but the TOS era installments were dominated by redshirt deaths, as the dominant uniform style featured all personnel wearing red. Star Trek Generations noted that crew losses from the destruction of the 1701-D were low. However, Star Trek II: The Wrath of Khan, Star Trek: First Contact, and Star Trek Nemesis all featured scores of battle-related crew deaths. As noted, the The Wrath of Khan losses were all redshirts, but the TNG losses were more varied, while continuing their pattern of conn officer attrition. It should also be noted that Spock and Kirk died in The Wrath of Khan and Generations, respectively – and in each movie, the officers wore red uniform shirts.
Star Trek: Deep Space Nine featured many security personnel – such as Ensign Paul Gordon in DS9: "Rocks and Shoals" – and engineers dying, sometimes in large numbers. However, while this maintains the tradition of TOS's most dangerous areas to work, these officers wore yellow shirts by this era. There was no pattern of redshirted crew loss until command officers were noted as dying in the war story arcs that dominated the last half of the series. However, the first known redshirt death on DS9 came when an unnamed officer was killed in Ops by the Cardassian counter-insurgency program in "Civil Defense". The redshirted conn officer of the USS Defiant also was killed during the ship's first battle with Dominion forces, in "The Search, Part I". In the Dominion War especially, entire fleets of starships were biting the dust, indiscriminate of shirt color.
In Star Trek: Voyager, the crew members who initially died on screen were wearing red uniforms, such as Stadi (who was another flight controller) and Cavit. However, over the seasons of Voyager, it became clear gold was a dangerous color in the Delta Quadrant, with most officer victims who died in the series doing so while clad in gold uniforms. Because USS Voyager had no way to replace crew, the only massive number of deaths took place in alternate timelines, with the two exceptions being the first episode, in which a large number of the senior staff were killed when the Caretaker's array displaced Voyager, and a situation in the second season episode "Deadlock" wherein both the crew and the ship were duplicated, with one of the Voyagers being destroyed not long after, along with its entire crew.
In Star Trek: Enterprise, more engineers and MACOs were lost than officers from any other division. In the case of the MACOs, this was consistent with their evolution into the Starfleet security forces. Both branches wore red as a department color (although MACOs seemed to wear splatter camouflage more than anything).
In the rebooted continuity beginning with the film Star Trek, all Starfleet Academy cadets wear red. However, Enterprise Chief Engineer Olson, prominently wearing a red space diving suit, became the first notable redshirt death in the alternate reality, as depicted in the aforementioned film.
Star Trek: Enterprise producer and writer Mike Sussman, a longtime fan himself, fulfilled a lifelong dream by putting on an original series red shirt to portray a dead crewman aboard the starship Defiant in "In a Mirror, Darkly, Part II". Sussman's trousers were the same ones worn by Gerrold during his DS9 cameo some eight years prior (Gerrold's name was stitched in them). Sussman's TOS-style boots had been worn previously by Avery Brooks.
"RedShirt" is the default player's name in the multi-player portion of the game Star Trek: Voyager - Elite Force.
The icon of the doomed redshirted crewman has translated to a number of other pop culture and literary media and parodies.
- In a Star Trek parody episode, "Where No Fan Has Gone Before", a character created to replace James Doohan as Montgomery Scott in the cast was named Welshie. He (or more specifically, the actor who played him) was killed, dismembered, and vaporized by three separate blasts from a cloud creature named Melllvar. In addition, the Democratic Order of Planets (D.O.O.P.) has a military force composed of men in red uniforms, who are often utilized as cannon fodder.
- During Season 5 when the Los Angeles headquarters for the Counter Terrorist Unit LA Domestic Unit is attacked with nerve gas, CTU security guards wearing red uniforms are among the dead.
- Captain Simian & the Space Monkeys
- Throughout the series red shirt wearing Holographic baboons called Holo-Boons are sent out ahead of the crew and usually end up destroyed.
- Family Guy
- An red-shirted ensign named Ricky was understandably frustrated at being assigned to the landing party for a "dangerous mission", which will likely result in the death of a member, along with Kirk, Spock, and Dr. McCoy - but he ended up surviving long enough to announce "I did not see that coming!" from out of a crowd gathered around for the death of William Shatner.
- Galaxy Quest
- Guy Fleegman (the actor who portrayed "Crewman Number 6" in one episode of the first television series) lamented in the middle of a crisis "I'm expendable! I'm the guy in the episode who dies to prove how serious the situation is!"
- At another point Gwen DeMarco (Lt. Tawny Madison) comments upon seeing a group of innocent-looking aliens turn into savage monsters "We'd better get out of here before one of those things kills Guy!"
- Guy survived the real-life adventure (in fact, he took the least amount of damage compared to the rest of his crewmates, such as being the only person on the bridge NOT shot by Sarris before the activation of the Omega 13), and became a regular cast member on the second incarnation of the show, portraying "Security Chief Roc Ingersol".
- Genki Wear
- Has a fragrance for men entitled Red Shirt. Its tag line: because tomorrow may never come.
- Kim Possible
- Kim, Ron, Dr. Drakken, and Shego are trapped in a reality created by TV cable shows. While jumping around the channel, Kim becomes a crewman of a Star Trek-like show. She talks to Wade (who's still in "reality"), and he reassures her that she'll be safe as long as she's not wearing a... Kim cuts him off, moaning, "... red shirt."
- South Park
- In episode "City on the Edge of Forever" (aka "Flashbacks") one of the children on a trapped school bus, on the edge of a cliff, waiting overnight for the bus driver to return with help, wore a red commander uniform, with an Enterprise assignment patch. He went outside the bus to scout around and was promptly eaten by a monster while TOS background music played. These elements of the episode tied into its being named for the TOS episode, "The City on the Edge of Forever".
- Boone told Locke that redshirts always get killed and Locke commented that Kirk "must have been a piss-poor captain." Ironically, Boone was the first cast member to die later that season. Additionally, Locke was played by Terry O'Quinn, who previously portrayed Erik Pressman, a former captain of dubious merit who had lost most of his crew (presumably including several redshirts) aboard the USS Pegasus.
- Robot Chicken
- In a twist on the standard, when the ship loses power to life support, Kirk, Spock, Uhura, Dr. McCoy, and Toby The Redshirt beam onto the nearest planet. The redshirt is the only member of a landing party to bring a phaser, and the only one to survive.
In another sketch a crew member is about to put on his redshirt upon hearing that they have met a species with various sexual advantages and disadvantages. After hearing one of the advantages he puts on the shirt and the door opens and a massive flame bursts in and incinerates him, a second later a voice announces that the ship is on fire.
- Space Quest
- Several references are made, since the series is a spoof on science fiction with strong Star Trek roots. In Space Quest I: The Sarien Encounter, nearly all people killed (including the player) are wearing red, though the good guys are more purple than red. In the VGA remake, it is humorous to note the Sariens are heavily armored... and the armor is red. In Space Quest V: The Next Mutation, Roger's security officer refuses to follow him into danger because his shirt is red and it's bad luck. Again, almost all people who die (or mutate) wear red.
- Stargate SG-1
- In an episode two characters hide in a cargo bay and one of them, played by Star Trek: Enterprise actor John Billingsley, becomes skeptical, saying, "We're all gonna die, we might as well wear red shirts..."
- Warhammer 40,000
- In the kill teams game, one of the wargear is named a redshirt. He has very low stats and the player will be rewarded if he lives until the end of the game.
- In the December 28, 2009 episode one of the myths taken on was the effectiveness (completely non-existent, as it turned out) of the makeshift cannon used by Kirk which would prove decisive during the battle with the Gorn in "Arena" - the crew outfitted the dummy (who would in fact be "injured" far more than his Gorn target) in a red shirt, since bad things usually happened to Trek characters who wore them.
- Think Geek
- The website Think Geek, has a red shirt for sale. It is a normal star trek design, save for the word "EXPENDABLE" writen in star trek font on the front.
- The Enterprise Blues Band
- A band led by Vaughn Armstrong, Casey Biggs, Richard Herd, William Jones, Ronald Moore, and Steve Rankin pay tribute with their song "Redshirt Boogie Blues". 
- Eek the Cat
- has a parody of Star Trek in which a "Red Shirt" Security Guard is killed after landing on a Alien Planet
- Criminal Minds
- In an episode, analyst Penelope Garcia says "Oh my God, she was doomed. Like Emily Brontë doomed. Like Shakespeare doomed. Like red-shirted ensign in Star Trek doomed."
- Gears of War
- In Gears of War, wearing a helmet is equivalent to wearing a red shirt in Star Trek. Specifically, the character Carmine (another name for a shade of red) dies on-screen, while another named Rojas (Spanish for red) is found dead early on.
- Warehouse 13
- "To him we are just..."
- "Redshirts ?"
- (unbaffled) "Yeah."
- "First, he doesn't think w're redshirts, and second, it's so cool that you knew what I meant."
Discussion between main characters Myka Bering and Pete Lattimer about their direct superior's propensity to keep them in the dark in first season's episode Implosion.
- In the episode The Princess and the Pear, psychologist Dr. Lance Sweets, dresses like a Red Shirt to infiltrate a convention and find out who the murderer is.
- The Suite Life on Deck
- The episode 'Starship Tipton' the gang goes to the future and have to wear outfits that look like the ones from Star Trek. Marcus gets a red one and hates it saying "The guys in red always get killed.", and then it shows a man in a red outfit who opens an airlock and gets sucked into space.
- How it Should Have Ended
- In the Star Trek episode of the internet cartoon series How It Should Have Ended, Kirk ordered all redshirts to be thrown out in space to make the enterprise light enough to escape the blackhole. The plan failed.
- World of Warcraft
- In the Twilight Highlands zone of the MMO, an Orc non-playable character (NPC) that is given to the player for the quest "Madness", when clicked, says the line "Hey, does this red shirt make me look expendable?" The "red-shirt" in question is actually the tabard with the colors and insignia of the Horde, one of the two major playable factions in the game. The NPC, funny enough, dies once you reach your destination, as the Warchief of the Dragonmaw kicks him into the fire. The NPC also delivers the quote "Once these negotiations are over, I look forward to a long prosperous life" enroute, as well, to foreshadow what untimely becomes of him. Players must own the expansion pack, "Cataclysm", to access the zone and this quest.