Hitstun
The brief window in which a struck character can do nothing at all — the grammar fighting games use for what is allowed to follow a hit.
When an attack lands, the opponent enters hitstun: a number of frames of forced helplessness. If the next attack's startup is shorter than the remaining hitstun it connects, and the two blows form a combo. If it is longer, the opponent regains control and can block or strike back.
The concept is therefore inseparable from frame data: the entire combo system is arithmetic on how many frames each move costs. Related states are blockstun (the same lock, but on block) and hitstop, the short freeze of both characters that gives a hit its weight. The mechanic dates back to the arcades and Street Fighter II, and lives on in every modern fighter.