The definition of an elbow is the joint that bends between the top and bottom parts of the arm, or anything bend in a 45° or 90° angle.(noun)
An example of an elbow is the part of the arm that bends when swinging a tennis racket.
See elbow in Webster's New World College Dictionary
Origin: ME elbowe < OE elboga < PGmc *alino-boga (> Ger ellenbogen): see ell & bow
See elbow in American Heritage Dictionary 4
Origin: Middle English elbowe
Origin: , from Old English elnboga; see el- in Indo-European roots.
90° angle elbow with female threads at both ends
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