The arm is the part of the human forelimb between the wrist and the shoulder. The upper arm bone (humerus) is attached at the shoulder by a ball-and-socket joint and to the lower arm by a hinge joint at the elbow The lower arm consists of the ulna and radius, which rotates over the ulna to turn the wrist. The chief arm muscles are the biceps and the triceps and the strong hand muscles.

 

In zoology, the term "arm" is commonly applied to a pectoral appendage of vertebrates when freed from the usual functions of support and locomotion. The radiating lobes of starfish are also called arms (or rays), and the term is applied, too, to the branches of the lophophore in brachiopods and to other special structures.