A combat term for a high-damage attack or hit that can kill most characters with "one shot", hence the name.
Most high-level and elite raid bosses usually have some kind of "one shot" attack. Usually you will need a strategy to avoid, debuff, or absorb these attacks via your Main Tank.
A more recent use of the term applied to instance runs. During a run, if the raid is able to kill a boss on the first attempt (no wipes), they have "one-shotted" the boss.
The term can also apply to the over-power-able Rogue class. If the Rogue has lots of +Critical Hit equipped, or just a lot of Attack Power, he may be able to "oneshot" his enemy with an [Ambush].
-Rogues can usually only oneshot Clothies.
-An extremely effective defense to this strategy is to bind your "Target Nearest Enemy Player" function to a key/key combination of your choosing, then spam it, while rotating your character in a circle and running around. This may be hard to do for some players, but it is much harder, and almost impossible, to be oneshot during this time. Or you could attack him as soon as you have him targeted, to, essentially, "steal" the first strike.
-Oneshotting is near impossible, unless there is a tremendous level difference or you have plenty of Strength as a Warrior, Agility as a Rogue, Hunter, Shaman or Cat Druid. Or Intellect and/or Spellpower as a "Clothie", without Ambush as a Rogue, or some other situational skill or move that either has a high critical hit rate or inflicts incredible damage