The Collaborative International Dictionary of English v.0.48:

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 Xa, An, Y2. See Suffix, n., 2, and Subindex.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Subscript \Sub"script\, n.
   Anything written below. --Bentley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subscript
    adj 1: written or printed below and to one side of another
           character [syn: subscript, inferior] [ant:
           adscript, superior, superscript]
    n 1: a character or symbol set or printed or written beneath or
         slightly below and to the side of another character [syn:
         subscript, inferior] [ant: superior, superscript]