gratuity
noun

- an amount of money given as a reward for service; a tip

Cambridge Dictionary

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Gratuity is just a fancy word for a tip, such as the money you give to a server in a restaurant. It stems from the Medieval Latin gratuitas, meaning free gift, and probably from the Latin gratuitus (free, freely given).

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PSYCHOLOGY

You may be interested that the placement of a ‘no tip’ button on a payment interface exploits a sense of guilt that the guest may not have otherwise felt and makes him/her more likely to leave a tip.

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OTHER USAGE

The adjective "gratuitous" means "not necessary" or "with no reasonable cause". A film containing a lot of violence for no apparent reason can be described as having "gratuitous violence".

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SYNONYMS

a little something, bonus, fringe benefit, perk, alms, benefaction, contribution, donation, largesse, offering, perquisite, present, recompense, reward, sweetener, tip, token

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Practice OWAD in a conversation:

"A GRATUITY in not necessary in this restaurant as there's a 20% service charge applied to every bill."