Plain English Meaning
The upright frame pieces on either side of an airplane's cabin door. They are part of the airplane's structure and help hold the door in place.
Full Definition
The vertical structural members of an airplane's cabin frame that form the sides of the door opening, providing structural strength to the fuselage and serving as anchor points for the door, hinges, latches, and surrounding cabin structure.
Context Anchor
Seen in safety and traffic-scanning discussions, where doorposts may block a small part of the pilot’s outside view.
Derivation
From the everyday English term for the upright posts framing a doorway in a building. In aviation it carries the same basic image -- the vertical frame around the door -- but applied to the airplane's cabin structure.