Hitbox

The "hitbox" is a property of some mobs that increases the range of spells and abilities targeting it to account for its size. The game measures ranges from center to center, so if large mobs did not have a hitbox, players in melee with them would disappear inside them. (Melee abilities have a 5-yard range.)

Area of effect checks also add the hitbox. As well, the AoE is often larger than the tooltip indicates; for example, Swell affects players "within 0 yards", which are almost everyone in melee range.

The hitbox is a property of the target; thus, if a boss with a 10-yard hitbox and a player both have attacks with a 30-yard range, the player can attack from 10 feet farther away than the boss can.

Onyxia has an extremely large hitbox; she is able to be attacked by melee at least 10 yards away from where her model actually ends.