Money left by customers in thanks to the waiter/waitress
Usually the majority of a server's income comes from tips, therefore a large tip is looked on with happiness, whereas a bad tip will criminalize you to the server, bartender, bus boy, and hostess.
Server: That 8 top had a $90 bill and left me a $6 tip
A sum of money or a small bonus given to someone who provides a service; commonly done in restaurants, bars, and coffee shops.
Tips are generally based on a percentage of the bill and a reward towards the quality of service. 15 - 20 percent is common for good service. Anything less, generally means poor service.