An ambush is a battle tactic where a character takes advantage of concealment or the element of surprise to attack unsuspecting characters from concealed positions, such as among dense underbrush or behind hilltops. Sometimes, an ambush can involve the exclusive or combined use of improvised explosive devices, that allow the attacking characters to hit the targeted characters while minimizing the risk of being exposed to return fire.
In most RPG games, an ambush will forbid all player actions, allowing the enemy to act first (example of indications: red swirl in MOTHER/EarthBound, swapped positions in Final Fantasy).