Medical Definition of arm
1a: a human upper limbb: the part of the human upper limb between the shoulder and the wrist; also : brachium
2a: the forelimb of a vertebrate other than a human beingb: a limb of an invertebrate animalc: any of the usually two parts of a chromosome lateral to the centromere
3: a group of subjects provided a particular treatment in a clinical trial <Since this trial did not include a radiation-only treatment arm, it has been questioned whether radiation therapy alone might be as effective as sequential chemotherapy and radiation therapy in preserving the larynx.—Everett E. Vokes et al., The New England Journal of Medicine, 21 Jan. 1993>
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