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 applies to instance runs, where a party manages to finish a boss, especially a difficult one, in a single attempt.
Probably a one shot.
See also the quest: 
[15] One Shot. One Kill.