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American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A joint made of two arms attached by a pivot shaped like an elbow, allowing force to be exerted at the ends of the arms as the joint is expanded.

Wiktionary

1. n. An elbow or knee joint, consisting of two bars so connected that they may be brought quite or nearly into a straight line, and made to produce great endwise pressure when brought into this position.

GNU Webster's 1913

1. v. an elbow or knee joint, consisting of two bars so connected that they may be brought quite or nearly into a straight line, and made to produce great endwise pressure, when any force is applied to bring them into this position.

WordNet 3.0

1. n. a joint made by two arms attached by a pivot; used to apply pressure at the two ends by straightening the joint