(1):

(n.) Anything written below.

(2):

(a.) Written below or underneath; as, iota subscript. (See under Iota.) Specifically (Math.), said of marks, figures, or letters (suffixes), written below and usually to the right of other letters to distinguish them; as, a, n, 2, in the symbols Xa, An, Y2. See Suffix, n., 2, and Subindex.