A doorjamb is the vertical portion of the frame onto which a door

Door

A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside of a space....

 is secured. The jamb bears the weight of the door through its hinge

Hinge

A hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...

s, and most types of door latches and deadbolt

Deadbolt

A dead bolt or dead lock , is a locking mechanism distinct from a spring bolt lock because a deadbolt cannot be moved to the open position except by rotating the lock cylinder. The more common spring bolt lock uses a spring to hold the bolt in place, allowing retraction by applying force to the...

s extend into a recess in the doorjamb when engaged, making the "true" (leveling) and strength of the doorjambs vitally important to the overall operational durability and security of the door.

The word "jamb" comes from the French "jambe", meaning "leg".