Reaction Fire

The enemy jumps out, the soldier squeezes the trigger.

In Turn Based Tactics/Turn-Based Strategy usually called "reaction fire", in turn-based RPG "Guard" action.

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.

Possible alternative names: "Reaction Attack Feature" or "They heard us coming" (if it's going to be expanded to a generic trope)

Examples of Reaction Fire include:

Tabletop Games

Alternity Warships specifies "defensive fire": before missiles, bombers and boarding pods resolve their attack, the defender gets to shoot them at point-blank range with all applicable weapons that didn't attack anything yet on this round.

Role Playing Game

Wizardry has "Parry/Guard" mode that allows a character to attack only if and when attacked by an enemy and rest a little otherwise.

Turn Based Tactics

Power Dolls has "opportunistic shooting". Only direct auto-fire ("D1") type weapons allow reaction firing at ground units, and only anti-air weapons at aircraft. After every step of an enemy unit in range, the window with list of weapons that can shoot pops if your unit retains enough Action Points and didn't fire yet - you can choose to wait until they are very close or even until another target is next to the first one to catch them in one burst. Of course, if enemies get too close, they may detect your units and shoot first.
X-COM series allow to reserve time units for reaction firing in Turn Based Tactics. It's resolved via Reaction stat multiplied by the fraction of TU left. When an unit shoots or walks, TUs are spend, effective Reaction value will eventually drop below the enemy unit's, and then that unit has initiative and may perform action - it spend TUs, which probably immediately drops its effective Reaction below the first unit, etc.
- 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 flechette loads, even it) and missile launcher. Given that it includes riot shotgun, flamethrower and 20x bullets long burst from LMG, this can be quite deadly. Reaction happens if the unit has Reaction Fire mode enabled, enough TUs for this shot reserved, and the enemy unit expends in its sight as many TUs as a shot takes - e.g. running across 1 square of a covered corridor is quicker than a snapshot, thus it doesn't trigger RF.
The new Shadowrun game from Harebrained Schemes includes an "overwatch" mode which is absolutely indispensible, especially in the final levels of the game.

Turn-Based Strategy

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 Feature "Move with reaction fire on". 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 - but the first submarine after taking damage once may pass right under it.
In Master of Orion if a ship couldn't use some weapons (other than Heavy or Special) during its turn, it will fire these weapons (unless they are disabled) at an approaching enemy. Also, there's defensive fire - Point Defence weapons not used during the last turn yet get the first shot at missiles or small craft attacking the ship.