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

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "honorarium":
   Trinkgeld, amends, atonement, blood money, bonus, bounty, bribe,
   compensation, consideration, damages, donative, double time,
   emolument, fee, gratuity, gravy, grease, guerdon, incentive pay,
   indemnification, indemnity, inducement, lagniappe, largess,
   liberality, meed, palm oil, pay, payment, perks, perquisite,
   pourboire, premium, price, quittance, recompense, redress,
   remuneration, reparation, requital, requitement, restitution,
   retribution, return, reward, salvage, salve, satisfaction,
   smart money, solatium, something extra, sportula, sweetener, tip,
   wergild



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

HONORARIUM. A recompense for services rendered. It is usually applied only
to the recompense given to persons whose business is connected with science;
as the fee paid to counsel.
     2. It is said this honorarium is purely voluntary, and differs from a
fee, which may be recovered by action. 5 Serg. & Rawle, 412; 3 Bl. Com. 28;
1 Chit. Rep. 38; 2 Atk. 332; but see 2 Penna. R. 75; 4 Watts' R. 334. Vide
Dalloz, Dict. h.t., and Salary. See Counsellor at law.