A nuke (nuclear strike) is achieved with 11 kills, or 18.5 assists, or a combination of both.
The nuke takes 25 seconds to detonate. The time is in the bottom right corner of the HUD.
An icon saying "KILL" will appear above players with nukes, revealing them no matter where you are. If the player arming the nuke is killed, the nuke will be cancelled. If the player stays alive until the time hits 0, there is no defence for this; every enemy dies with the nuker, no matter what.
This can be very useful in the Zombies mode.