1. A small dish of food or drink served at the beginning of a meal to stimulate the appetite; usually, served before the main part of a meal or as the first course.
2. A sample of something that is meant to stimulate an interest.
3. Mainly in the U.S., the first part of a meal: "The menu showed what the average cost of a full three-course meal would be; including an appetizer, the main course, and the dessert."