Nuke

A nuke can mean different things, all related to the phenomenon called "nuking" (see below).

As a noun
As a noun, it refers to an attack - either magical or physical - which is much more powerful than a "normal" attack.
 * Usually an attack that deals direct damage (such as Fireball or Wrath) rather than a DoT.
 * A player performing the role of DPS (see Damage (role)) is sometimes referred to as a "nuker".

As a verb
As a verb (to / will nuke, am / is / are nuking, has / have nuked), it refers to a combat tactic in which the player attempts to gush out the highest amount of damage possible.
 * Nuking
 * "Overnuking" is where a player deals so much damage (therefore generating excess threat) that it pulls aggro away from the tank.