The joint between the upper arm and the forearm.

(by extension) Any turn or bend like that of the elbow, in a wall, building, coastline, etc.; an angular or jointed part of any structure, such as the raised arm of a chair or sofa, or a short pipe fitting, turning at an angle or bent.

“the sides of windows, where the jamb makes an elbow with the window back”

A detective.

To push with the elbow.

“He elbowed his way through the crowd.”

(by extension) To nudge, jostle or push.