1.n. a reference point, standard, or indicator. 2.n. a variable that is employed to indicate the presence of another phenomenon or event. 3.n. a number formed from a combination of other measures to represent another, more general entity. For example, an index of a person’s graduate school potential might be formed from his or her undergraduate grade point average and score on an admissions test. 4.vb. to act as an index of.