1. In anatomy, the upper extremity from shoulder to elbow. In ordinary usage, arm means the entire upper extremity, from shoulder to hand.

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MUSCLES OF THE ARM Anterior and posterior views.
2. In clinical experimentation or research science, a treatment protocol in which subjects are enrolled.
3. In research on a therapeutic agent, one of several possible interventions. Most clinical trials include an active treatment arm, in which participants are exposed to the agent that is under study, and a placebo arm, a sham therapy used for the purpose of contrast or comparison.

articulated arm

A jointed instrument or positioning device used in imaging and in therapeutic procedures, as for permitting stereotactic localization of deep anatomical structures, guiding the collection of ultrasonic images, or focusing or directing laser energy.

Boston arm

SEE: Boston arm

brawny arm

A hard, swollen arm caused by lymphedema after mastectomy.

Saturday-night arm

A colloquial term for musculospiral paralysis.