A gratuity is a tip that's given by a customer in exchange for services. You might leave a gratuity of several dollars when you eat lunch in a restaurant or when you get your hair cut.
A gratuity is usually a relatively small amount of money, a percentage of the total cost of a purchase or service. A gratuity doesn't pay for the goods purchased — it's a separate reward to the person who did the work. For example, in the United States, waiters and waitresses typically expect a gratuity of between 18 and 20 percent of the total cost of a meal.