The Collaborative International Dictionary of English v.0.48:

Honorarium \Hon`o*ra"ri*um\, Honorary \Hon"or*a*ry\, n. [L.
   honorarium (sc. donum), fr. honorarius. See Honorary, a.]
   1. A fee offered to professional men for their services; as,
      an honorarium of one thousand dollars. --S. Longfellow.
      [1913 Webster]

   2. (Law) An honorary payment, usually in recognition of
      services for which it is not usual or not lawful to assign
      a fixed business price. --Heumann.
      [1913 Webster]

WordNet (r) 3.0 (2006):

honorarium
    n 1: a fee paid for a nominally free service