olecranon

n. part of the forearm which joins with the upper arm at the elbow forming a bony protrusion (Anatomy)

The olecranon is a large, thick, curved eminence, situated at the upper and back part of the ulna. It is bent forward at the summit so as to present a prominent lip which is received into the olecranon fossa of the humerus in extension of the forearm. Its base is contracted where it joins the body and the narrowest part of the upper end of the ulna.
See more at Wikipedia.org...

Olecranon /olec·ra·non/ bony projection of the ulna at the elbow.olec´ranal o·lec·ra·non n. The large process on the upper end of the ulna that [more]Olecranon - Community and Resources

Noun
1. process of the ulna that forms the outer bump of the elbow and fits into the fossa of the humerus when the arm is extended
(synonym) olecranon process
(hypernym) process, outgrowth, appendage
(part-holonym) ulna, elbow bone

(n.)
The large process at the proximal end of the ulna which projects behind the articulation with the humerus and forms the bony prominence of the elbow.