Medical Definition of index
plural indexes or indices \-də-ˌsēz\play
1: index finger
2: a list (as of bibliographical information or citations to a body of literature) arranged usually in alphabetical order of some specified datum (as author, subject, or keyword) <Index Medicus of the United States National Library of Medicine>
3a: a ratio or other number derived from a series of observations and used as an indicator or measure (as of a condition, property, or phenomenon) <physiochemical indexes of the urine, the blood, and the gastric juice—Journal of the American Medical Association>b: the ratio of one dimension of a thing (as an anatomical structure) to another dimension—see cephalic index, cranial index
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