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.