The Collaborative International Dictionary of English v.0.48:

Proponent \Pro*po"nent\, a. [L. proponens, p. pr.]
   Making proposals; proposing.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Proponent \Pro*po"nent\, n.
   [1913 Webster]
   1. One who makes a proposal, or lays down a proposition.
      --Dryden.
      [1913 Webster]

   2. (Law) The propounder of a thing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

proponent
    n 1: a person who pleads for a cause or propounds an idea [syn:
         advocate, advocator, proponent, exponent]