The Collaborative International Dictionary of English v.0.48:

Prolocutor \Prol`o*cu"tor\, n. [L., from proloqui, p. p.
   prolocutus, to speak out; pro for + loqui to speak.]
   [1913 Webster]
   1. One who speaks for another. --Jeffrey.
      [1913 Webster]

   2. The presiding officer of a convocation. --Macaulay.
      [1913 Webster]

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

PROLOCUTOR. In the ecclesiastical law, signifies a president or chairman of
a convocation.