Killstreak

A killstreak is when a player gets two or more kills in a row without dying. In Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Modern Warfare 2 the player is given various rewards depending on the length of the killstreak.

Call of Duty 4: Modern Warfare

 * 3 - UAV Recon: Deploys a UAV to scan the map for 30 seconds and reveal enemy locations (enemies with the second tier perk UAV Jammer will not be scanned by the UAV).
 * 5 - Airstrike: Initiates an airstrike on a targeted area.
 * 7 - Attack Helicopter: Deploys either an Mi-28 hind or an AH-1 Cobra, depending on what team you are on, to attack enemy players for 1 minute.

Call of Duty: World at War

 * 3 - Recon Plane: Same effect as UAV Recon.
 * 5 - Artillery Strike: Calls in artillery shells to strike a designated location.
 * 7 - Dogs: Releases several vicious dogs for one minute that attack enemy players.

Call of Duty: Modern Warfare 2
In this game, players can choose up to three killstreak rewards they can earn, rather than just the standard three. At the start, the player can only use UAV, Care Package and Predator Missile, with the option to unlock one new killstreak at each of the following levels: 10, 15, 23, 35, 47, 51, 55, 59, 63, 66, 68, and 69 (all killstreak rewards will be unlocked at lvl 69). When a killstreak of 8 or more is activated, a message will notify all players in the match, showing the callsign and the streak being used (except in Hardcore or when the killstreak is a Stealth Bomber being deployed by the opposing team). It is worth noting that two killstreaks that share the same killstreak amount cannot be used together, one must be picked over the other. For example, for the 7 kills slot the player can use either the Harrier Strike or the Attack Helicopter, but not both.

In non-Hardcore game modes, the calling of a killstreak of 8 or more (Emergency Airdrop and up) will be announced with the player's callsign in the top-right corner, with the name of the killstreak directly below it (except for the Stealth Bomber and Tactical Nuke, which is not announced to the enemy team). Also, even though a Tactical Nuke is announced to both teams, the player's callsign on the top-right corner will only be shown to players on that player's team.

For controllable killstreaks (Predator Missile, Chopper Gunner and AC-130), if the player is killed while operating the killstreak, the player can continue using the killstreak until it is done and then respawn or enter spectator mode as normal. The only exception to this is Search and Destroy. This outcome has been protested by some who feel the killstreak should end if the player dies. Note: The number of kills required to obtain a particular killstreak reward can be reduced by one when using the Hardline Perk.

Call of Duty: Black Ops
Not much is known about the killstreaks featured in Call of Duty: Black Ops, although it will feature the same customizable killstreak reward system as Modern Warfare 2. Killstreaks will no longer contribute to higher killstreaks like in previous titles. Also, the Tactical Nuke or other game-ending killstreak will not appear.

Trivia

 * In Call of Duty 4: Modern Warfare, and Call of Duty: World At War, killstreaks do not stack, meaning that once a killstreak reward is available it replaces any unused killstreak reward. In Call of Duty: Modern Warfare 2 they stack with the most recent rewards earned being first in the queue.
 * Killstreaks in Call of Duty: Modern Warfare 2 that do count towards other killstreaks will only do so during the same life. For example, a Predator Missile can contribute towards any other killstreak the player has selected of 6 or higher, but if used after the player dies, it will not count towards any killstreaks. By definition, a killstreak cannot contribute towards a lower one. Killstreaks from care package and emergency airdrop don't count towards your current streak.
 * In World at War there originally was 4th Killstreak and Carpet Bomber killstreaks.
 * In Modern Warfare 2, the UAV, Counter UAV and EMP are the only killstreaks that do not directly kill enemy players.
 * In Modern Warfare 2 there are chatter sounds for Little Bird and Airdrop (not Care Package or Emergency Airdrop) killstreaks, however, these killstreaks are cut from final game. Also, chatter sounds for enemy Predator Missile, Sentry Gun, and Airstrike can be found in game files, but they're not used.
 * In Modern Warfare 2, if you switch teams on a private match after you have earned a killstreak without using the killstreak first, you will lose the killstreak.
 * An EMP should technically last for much more than a minute. It should disable electronics for at least a full length match and a range equal to the size of the United States. Although this was probably an intentional mistake so that the EMP wasn't too cheap.