Hot Box

From UltimateMK.com

A Hot Box is the nickname for an invisible box that determines what part of a character's sprite will cause the attack to connect if touched to the hittable area or Cold Box of an opponent. For instance, when a character punches, there is a Hot Box around the character's arm when the punch is fully extended.

In the 2D Mortal Kombat games, characters have one Hot Box at a time, just like Cold Boxes, which they also have 1 of.