An atomic weapon (also known as an atomic bomb, A-bomb, nuclear weapon, or more commonly nuke or nuclear missile) was a term that generally described a device developed by Humans and other species that utilized the principles of either or both of nuclear fission and nuclear fusion to release massive destructive energies. A relatively low-yield atomic weapon could, for example, destroy a small city. The near-ground detonation of a nuclear weapon could produce a mushroom-shaped cloud and the subsequent radioactive contamination of the surrounding areas could cause radiation sickness and other health problems. Sufficient use of atomic weapons could also result in a sudden reduction in overall planetary temperatures due to particulate matter in the atmosphere, called a "nuclear winter". (TNG: "A Matter of Time"; VOY: "Dragon's Teeth", "Friendship One")
Many cultures have used atomic weapons. Vulcans developed and used nuclear weapons widely in the wars leadings up to the Time of Awakening around the year 350, devastating their civilization. (ENT: "Awakening") The Talosian society was effectively destroyed by the use of atomic weapons. (TOS: "The Cage") A jet plane from the United States of America on Earth that was sent to intercept the USS Enterprise in 1969 was thought by Spock to potentially have missiles armed with nuclear warheads. (TOS: "Tomorrow is Yesterday") The planet Zeon fired a thermonuclear missile at the USS Enterprise in 2268. (TOS: "Patterns of Force") Weapons considered by 23rd century Starfleet to be "atomic" (possibly atomic bombs) were used in the Earth-Romulan War. (TOS: "Balance of Terror") Troyian used nuclear side-arms. (TOS: "Elaan of Troyius")
Earth, especially, saw both development and use of atomic weapons. Combat usage was at first confined for over one hundred years to only two nuclear weapons used by the United States of America on Japan at the end of the Second World War, including the bomb, code-named, "Fat Man". Because of the weapon's destructive power, though, countless amounts of nuclear weapons were produced and tested (such as at Bikini Atoll) between 1945-1990, mostly by the US and the Soviet Union. (ENT: "Proving Ground") During this period, which became known as the Cold War, mankind was constantly on the brink of self-annihilation through, among other atomic weapon delivery systems, orbiting nuclear devices. (TOS: "Assignment: Earth"; ENT: "Carbon Creek")
Quark, Rom, Nog, and Odo were able to experience a test of an atomic weapon when they were accidentally transported to 1947 Earth. There they harnessed the beta radiation released by an atomic bomb test in the Nevada desert to return to the 24th century. The Ferengi never tested atomic weapons on their own planet, much less actually used them in combat, and Quark was shocked at the "stupidity" of Humans for polluting their own planet with atomic testing. (DS9: "Little Green Men")
Even though peace was declared between those two nations at the end of the 20th century, Humanity's fear of nuclear holocaust was nonetheless realized beginning in circa 2026 with the Third World War. Six hundred million were killed and major cities were destroyed, many through use of atomic weapons. (Star Trek: First Contact) This led to the post-atomic horror. (TNG: "Encounter at Farpoint") But in a great irony, Zefram Cochrane's first successful faster than light vessel manned by Humans was launched using the remnants of a nuclear missile. (Star Trek: First Contact)
By the mid 23rd century, advances in deflector shield technology led to nuclear weapons losing their tactical significance in ship-to-ship combat. Romulan Birds-of-Prey employed them solely as self-destruct devices, and such a warhead detonated point blank in front of a starship caused relatively light casualties as well as temporary electrical disruption. (TOS: "Balance of Terror") Low-yield nuclear weapons were still used such as cobalt bombs, that produced an explosion 1/10,000th of the yield of an ounce of antimatter. (TOS: "Obsession")
In the game Star Trek: 25th Anniversary, some time in the 1260s a conflict between the rival societies, the capitalist-like Lucrs and the Communist-like Sofs led to a war with atomic weapons, which devastated their planet, Alpha Proxima III.