The Collaborative International Dictionary of English v.0.48:

Exponent \Ex*po"nent\, n. [L. exponens, -entis, p. pr. of
   exponere to put out, set forth, expose. See Expound.]
   1. (Alg.) A number, letter, or any quantity written on the
      right hand of and above another quantity, and denoting how
      many times the latter is repeated as a factor to produce
      the power indicated;

   Note: thus a^2 denotes the second power, and a^x the xth
         power, of a (2 and x being the exponents). A fractional
         exponent, or index, is used to denote the root of a
         quantity. Thus, a^1/3 denotes the third or cube root
         of a.
         [1913 Webster]

   2. One who, or that which, stands as an index or
      representative; as, the leader of a party is the exponent
      of its principles.
      [1913 Webster]

   3. one who explains, expounds, or interprets.
      [PJC]

   Exponent of a ratio, the quotient arising when the
      antecedent is divided by the consequent; thus, 6 is the
      exponent of the ratio of 30 to 5. [R.]
      [1913 Webster]