Stealth is a form of technology or ability to render starships, personnel or technology invisible, or at least undetectable to sensors.
Many spacefaring civilizations used varying forms of stealth technology. Perhaps the epitome of stealth was the cloaking device, which used selective bending of light and emission dampening to render a ship totally invisible and undetectable. This was used by many organizations, including the Romulan Star Empire, the Klingon Defense Force, and the Suliban Cabal. However, the power consumptions of cloaking technology were high, meaning weapons and shields could not be used while a cloak was on in the early stages of the technology. (TOS: "Balance of Terror"; Star Trek III: The Search for Spock, Star Trek VI: The Undiscovered Country, Star Trek Nemesis)
The holy grail of cloaking technology was a phasing cloak, which allowed a starship to pass through solid matter and made it impervious to conventional weapons. (TNG: "The Pegasus") Other technology could be used to achieve the same effect, such as subspace force fields with adjusted synchronic distortion, interphase generators or other interphase devices. These systems could be used to alter the phase variance of matter and energy to disappear from the perceptual range. (TNG: "The Next Phase", "Time's Arrow"; VOY: "Distant Origin", "Scientific Method")
More primitive versions of stealth were known to be used by many other races, such as the stealth technologies used by the Xyrillians, the Hirogen, and the Kazon-Nistrim (called "masking circuitry"). (ENT: "Unexpected"; VOY: "State of Flux", "Flesh and Blood") The people of Cheron had an unusual yet advanced form of stealth technology. Their ships were perfectly detectable by sensors, but sheathed in special materials that made them completely invisible. (TOS: "Let That Be Your Last Battlefield") Subspace could also be used as a place of concealment. The Dominion used anti-personnel mines that hid in subspace and appeared at random to normal space. (DS9: "The Siege of AR-558") The "Think Tank" also had the ability to submerge into subspace to hide. (VOY: "Think Tank")
Stealth technology might also include the use of holographic equipment and was not limited to space travel. In 2154, the Romulans deployed two drone-ships that used holographic technology to disguise themselves as other vessels. (ENT: "Babel One", "United", "The Aenar") The Federation used holographic technology to hide facilities called duck blinds from pre-warp civilizations (TNG: "Who Watches The Watchers") and isolation suits to hide the personnel on these assignments. (Star Trek: Insurrection) Vosk (leader of the Na'kuhl) developed a form of stealth time travel that enabled him to escape Daniels' forces in the 29th century to an alternate 1944 Earth. (ENT: "Storm Front", "Storm Front, Part II") The inverter used by the Ansata terrorists on Rutia IV and the subspace transporter used by Bok were methods of stealth transporting, in which it was possible to beam though deflector shields and extremely difficult for others to determine the origin and destination of the transport. (TNG: "The High Ground", "Bloodlines")
Some species even possessed a biological ability for invisibility, such as the Tosk and the Jem'Hadar who had the ability to "shroud" themselves in a camouflage that acted as a personal cloaking field. This effectively hid them and their weapons from the naked eye and in the case of the Jem'Hadar from sensors as well. (DS9: "Captive Pursuit", "The Jem'Hadar", "The Abandoned") The Devidians could shift their own phase variance (TNG: "Time's Arrow") and the parasitic interphasic organisms from Thanatos VII lived in a natural state of interphase. While they could attach to their victims, they themselves couldn't be seen or touched. (TNG: "Phantasms") Other species had a chameleon-like ability to appear invisible such as the species native to Tarchannen III who produced a mimetic radiant effect in their skin and some of the genetically enhanced Suliban who possessed a similar ability from subcutaneous pigment sacs and the bio-mimetic garments they wore. (TNG: "Identity Crisis"; ENT: "Broken Bow")
The changelings were a shapeshifting species, that could use their morphogenic enzymes to appear as fire, fog, inanimate objects, flowers, trees or harmless small animals to avoid detection. (DS9: "The Search, Part II", "Things Past", "Chimera") Members of Fantome's species had the natural ability to refract their life signs, making them undetectable to sensors. (VOY: "The Void")