Source : Webster's Revised Unabridged Dictionary (1913)

Subscript \Sub"script\, a. [L. subscriptus, p. p. See
   {Subscribe}.]
   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 X_{a}, A_{n}, Y_{2}. See {Suffix}, n., 2, and
   {Subindex}.

Subscript \Sub"script\, n.
   Anything written below. --Bentley.

Source : WordNet®

subscript
     adj : written or printed below and to one side of another
           character [syn: {inferior}] [ant: {adscript}, {superscript}]

subscript
     n : a character or symbol set or printed or written beneath or
         slightly below and to the side of another character [syn:
          {inferior}] [ant: {superscript}]