Parry window
The short span of frames during which a parry input counters an attack instead of absorbing it.
The window is measured in frames at the game's tick rate, usually 60 Hz, and is often just a couple of frames wide — a button press right before the hit lands. Street Fighter III: 3rd Strike (1999) made parrying the centre of its defence, and Sekiro (2019) built its entire swordplay on a deflect window. The tighter the window, the higher the skill ceiling and the greater the reward for nailing it.
Designers balance it against the attack's frame data and against how much input buffering the game allows: a generous window forgives, a one-frame window separates masters from novices. Kin to invincibility frames and hitstop.