Hitbox
Also said as Hurtbox, Collision box
The invisible shape the game actually checks for contact. Strictly, a hitbox deals damage and a hurtbox receives it. Neither has to match the art.
Players blame themselves for deaths that seem fair and blame the game for those that do not. Making the player's hurtbox a bit smaller than the sprite is an easy way to get more of the first kind.
Say it
“Shrink the player's hurtbox to 70% of the sprite, and make enemy hitboxes a little smaller than their art, so near misses count as misses.”
Where you’ve seen it
In shoot 'em ups such as Touhou Project, the player's hit area is a tiny dot at the centre of the ship, which is what makes dodging through dense bullets possible.
Not to be confused with hurtbox
A hitbox is the part that hits you. A hurtbox is the part of you that can be hit. People say hitbox for both.