Ulna n. : The postaxial bone of the forearm, or branchium, corresponding to the fibula of the hind limb. See Radius.

Ulna n. : An ell; also, a yard.

Example:
How to differentiate radius and Ulna in a bone diagram?

Taxonomy
ulna, Origin, radius, bone, radius and ulna, Coronoid process, interosseous membrane, Olecranon process, trabeculæ, shaft, shaft of ulna, free encyclopedia, compact, distal ulna, olecranon, compact bone, medial epicondyle of the humerus, upper extremity.

Defination from the web:
ulna is broader proximally, and narrower distally. Proximally, the ulna has a bony process, the olecranon process, a hook-like structure that ... Posterior surface of ulna (also shares origin with the posterior surface of the radius bone) Extensor pollicis longus muscle. Origin ...