A “gratuity” is a gift or reward for service. Depending on where you are, a gratuity might also be called a “tip” or a “pourboire.” This last word comes from the French for “to have a drink” and refers to the coins you leave on the table for your server, so that they might get a pint for themselves after they’re off work.

Example:  “Restaurants often add an automatic gratuity of eighteen percent for large groups of people at one table; perhaps they think it makes it easier for their customers, but I rather resent it when the service has been poor.”