The Nuke is a gameplay mechanic added in update v0.9.9.99.1. It fits in the third killstreak slot.

The Nuke is awarded for getting a streak of 25 kills without dying in the same match. Notably, you can die after you get 25 kills, but still keep the ability to activate the nuke. It should also be noted that you can get multiple nukes and stack them, because nukes are obtained every 25 killstreak without dying. 

Fun Fact: The Nuke was most likely inspired by a similar feature, also named Nuke, from the Call of Duty series of games.

A HUD element seen in the image above appears next to your weapon's ammo count once you have pulled off the 25 killstreak. Pressing 3 (unless you have changed the setting in your keybinds) will activate a 10-second countdown, after which a white flash occurs and kills all enemy players on the map. The player is left unharmed. If they are playing a team mode, their allies will be left unharmed too. Many players get last-minute nukes and have less than 10 seconds to activate the nuke. Before update 3.3.4, the people that had less than 10 seconds to nuke did not get the nuke. However. as of update 3.3.4, Nukes automatically go off at the end of the game and get counted for points and stats.

Killing a player with the nuke awards 50 points, so the theoretical maximum score from a single nuke in a public FFA game (7 players) is 300, while in a custom game (16 players if hosted by a premium account holder) the maximum score is 750, if in a 20 player custom lobby hosted by a verified account holder then 950 and in a 40 person custom game the maximum score would be 1,950 per nuke.

The easiest mode to get the nuke in is Infect, because the zombies often rush you so if you have a strategic spot, you can farm kills on incoming zombies.

The Nuke does NOT end the game, and you are able to earn multiple nukes per round. For example, you get a nuke at a 25 kill streak, one at a 50 kill streak, and one at 75 kill streak. Additionally, you are still able to get nukes after dying after the first killstreak.

As a little side note, if you are playing the Infect game mode, and get 25 kills, then die, you can still use the nuke, killing all of the living players. It is known by some players as "the reverse nuke."

Nukes will only count on your account if and only if you are playing in a public lobby. Nukes in Custom Servers wont count neither in Challenges nor in your Official Statistics.

Only nukes which are called before the end of the game counts towards challenges and Official Statistics in Public lobbies.

All assets of the game can be downloaded here