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]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "bonus":
   Trinkgeld, balance, bonus system, bounty, bribe, buried treasure,
   compensation, consideration, decoration, discovery, dividend,
   donative, double time, extra, extra added attraction, extra dash,
   fee, filigree, filling, fillip, find, finding, flourish, foundling,
   frill, fringe benefit, gratuity, gravy, grease, hand-out,
   honorarium, incentive pay, inducement, lagniappe, largess,
   largesse, leftover, liberality, margin, ornament, overage,
   overmeasure, overplus, overrun, overset, overstock, oversupply,
   overtime pay, padding, palm oil, perk, perks, perquisite,
   perquisites, plus, pourboire, premium, remainder, remuneration,
   reward, salve, solatium, something extra, spare, sportula,
   stuffing, superaddition, surplus, surplusage, sweetener, tip,
   treasure trove, trimming, trouvaille, trove, twist, waifs,
   waifs and strays, windfall, windfall money, windfall profit,
   wrinkle



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

BONUS, contrads. A premium paid to a grantor or vendor; as, e. g. the bank
paid a bonus to the state for its charter. A consideration given for what is
received.