when used as a verb:
when a shot/attack that should've connected and appears to have connected... apparently doesn't.
A notoriousexample: TF2's melee hitreg is completely busted. You'll hear the melee attack connect but there'll be no hit sound and no damage will be done.
Short for hit registration.
generally used in TF2's weapons(eg. Shotgun, Sniper Rifle, etc.)
used for registrating the bullet IF IT DID HIT OR NOT
also used as a excuse when the player dies from a good person