noun
gratuity
gruh-TOO-uh-tee/ɡrəˈtuːəti/
A payment given beyond what is required, especially to thank someone for service.
The diner leaves a gratuity beside the bill.

Etymology
Gratuitas meant a “free gift,” which connects a gratuity with voluntary giving rather than a required charge. The English word came to name money given for a favor or service.
In context
Here in the United States, we're expected to tip people for good service. Standard gratuity for a server at a restaurant is 15 to 20%.
More examples
- She left a generous gratuity after the meal.
- The guide politely declined the offered gratuity.
- The policy explained when gratuities could be accepted.
Forms
gratuity · gratuities