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

• Found in most Beat Em Ups and Fighting Games.
  • In Final Fight, your best bet to break out of hit stun is to do a crowd control/ Extra Joy move.
  • This apparently accidentally invented Combos in Street Fighter II
• In Ehrgeiz's Forsaken Dungeon, most attacks from the mooks in the later dungeons will send you into 1 to 2 second long hitsun animations, thereby requiring you to use the shield attack.
• Mooks in Streets Of Rage have absolutely no qualms about ganging up on you. The game may give you some odd glitches in the most extreme cases.
• Lord help you if you play as Sentinel and get struck while flying. Sentinel has no pushback on his hitstun.

Type 2:

• In the classic entries of the Castlevania series, the Belmonts had the tendency of jumping back when hit. This usually means death if there's a bottomless pit right behind you.
• This is also the case with Zelda II. Use extreme caution when around lava.