An invincible hitbox is one kind of hitbox.
During an invincibility time, even if one’s body makes contact with an attack hitbox, he will not take the attack.
There are plenty of cases where only the upper or lower half of the body is covered, or the invincibility only applies to strike or throw hitboxes; in these cases, it is best to refer to the type of invincibility: “upper-body invincibility”, “foot-level invincibility”, “throw invincibility”, “full-body invincbility”, “complete invincibility”, etc.
Invincibility is rarely found in 3D fighting games.