Reaction Fire

The enemy jumps out, squeeze the trigger.

A very useful feature in various turn-based games that makes units less of sitting ducks when it's not their turn to act. Usually can be turned off, especially when friendly fire is possible. Sometimes can be set up to respond on a new target in range or only to an attack.

Role Playing Game

 * Wizardry allows a character to attack only if attacked via "Parry / Guard" mode.

Turn Based Tactics

 * Advanced Strategic Command has different number of reaction fire shots per turn for each weapon - sometimes none (e.g. a bomber will not attack any hostile unit walking by below) and sometimes multiple shots (defensive units like MG Nest), while anything can actively attack at most once. Most units stop moving when reaction fire is turned on (though still can attack anything in range) and wait until the next round before moving after it's turned off, others (like warships and Anti-Air) got "Move with reaction fire on" Feature. However, each new unit walking into range can be attacked this way only once, thus e.g. submarine destroyer will fire a range 3 torpedo at the first submarine, then range 2 bomb at the second, then range 1 bomb at the third - even if the first submarine after taking damage passed right under it.
 * Power Dolls pops the reaction fire window on every step of an enemy unit moving in range, if your unit got Action Points - so you can choose to wait until they are very close or until the second unit walks by to hit both with one burst - but then they may detect you and shoot first. However, only direct auto-fire (D1) type weapons allow reaction firing at all.
 * X-COM series allow to reserve time units for reaction firing in Turn Based Tactics modes. X-COM Apocalypse also retains it in Real Time with Pause modes (cityscape and tactics) has 3 possible aggression levels for attacks without player input (Never / On sight / In responce).
 * UFO Alien Invasion allows this for all weapons other than hand grenades, grenade launcher (and with pseudo-shotgun loads, even it) and missile launcher. Given that this includes up to 7x flechette shells shot from a riot shotgun or 20x bullets long burst from LMG, this can be quite deadly.