A door jamb is the vertical portion of the frame onto which a door is secured. Most types of door fasteners and deadbolts extend into a recess in the door jamb when engaged, making the strength of the door jambs vitally important to the overall security of the door.
In closets, it is possible to incorporate an electrical trigger in or on the jamb, controlling a light fixture in the closet so it turns on when the door is open, similar to the device found in a refrigerator.
The word "jamb" comes from the French "jambe", meaning "leg".
I was crouched by the doorjamb, gun straight out, scanning the room.