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.