The Collaborative International Dictionary of English v.0.48:

Deponent \De*po"nent\, n. [L. deponenes, -entis, laying down.
   See Depone, v. t.]
   1. (Law) One who deposes or testifies under oath; one who
      gives evidence; usually, one who testifies in writing.
      [1913 Webster]

   2. (Gr. & Lat. Gram.) A deponent verb.

   Syn: Deponent, Affiant.

   Usage: These are legal terms describing a person who makes a
          written declaration under oath, with a view to
          establish certain facts. An affiant is one who makes
          an affidavit, or declaration under oath, in order to
          establish the truth of what he says. A deponenet is
          one who makes a deposition, or gives written testimony
          under oath, to be used in the trial of some case
          before a court of justice. See under Deposition.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Deponent \De*po"nent\, a. [L. deponens, -entis, laying down (its
   proper passive meaning), p. pr. of deponere: cf. F.
   d['e]ponent. See Depone.] (Gram.)
   Having a passive form with an active meaning, as certain
   latin and Greek verbs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deponent
    n 1: a person who testifies or gives a deposition [syn:
         testifier, deponent, deposer]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DEPONENT, witness. One who gives information, on oath or affirmation,
respecting some facts known to him, before a magistrate he who makes a
deposition.