A character is paralized out of pain
Whenever a video game character is struck, he has the tendency of freezing from the attack, be it from a nice solid haymaker or a mere slap in the face. There are two types of hitstun animations: Type 1: The character is left wide open after the attack, has no way to break out of it on his own and is open for many more hits. This is what makes Combos possible.
Type 2: A recoil animation, whereas you lose control of your character for a small window of time. This usually means knockdown. The opposite of this is Mercy Invincibility, though there may be some overlap with type 2. Needs A Better Description. Examples: Type 1