The Collaborative International Dictionary of English v.0.48:

Bonus \Bo"nus\ (b[=o]"n[u^]s), n.; pl. Bonuses (-[e^]z). [L.
   bonus good. Cf. Bonny.]
   1. (Law) A premium given for a loan, or for a charter or
      other privilege granted to a company; as the bank paid a
      bonus for its charter. --Bouvier.
      [1913 Webster]

   2. An extra dividend to the shareholders of a joint stock
      company, out of accumulated profits.
      [1913 Webster]

   3. Money paid in addition to a stated compensation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bonus
    n 1: anything that tends to arouse; "his approval was an added
         fillip" [syn: bonus, fillip]
    2: an additional payment (or other remuneration) to employees as
       a means of increasing output [syn: bonus, incentive]