Gratuity

Noun

 * 1) A reward, service, or payment provided freely, without obligation.
 * 2) (common usage) An additional charge placed for services rendered; see also: service fee.

Synonyms

 * contribution
 * cumshaw
 * donation
 * gift
 * grace

Related terms

 * gratuitous
 * gratuitously

Thesaurus
Trinkgeld, balance, bonus, boodle, bounty, bribe, bribe money, complimentariness, consideration, costlessness, dividend, donative, double time, expenselessness, extra, fee, free ride, freebie, freeness, gift, gratification, gratuitousness, gravy, grease, honorarium, hush money, incentive pay, inducement, labor of love, lagniappe, largess, leftover, liberality, margin, no charge, overage, overmeasure, overplus, overrun, overset, overstock, oversupply, palm, palm oil, payoff, payola, perks, perquisite, plus, pourboire, premium, protection, remainder, reward, salve, something extra, sop, spare, sportula, surplus, surplusage, sweetener, tip

Etymology
From gratuite: (French gratuité:) <  gratuitas: <  gratuitus:; see gratuitous:.