Vegetarianism is the practice of following a plant-based diet including fruits, vegetables, cereal grains, nuts, and seeds, with or without dairy products and eggs. A vegetarian does not eat meat, including red meat, game, poultry, fish, crustacea, and shellfish, and may also abstain from by-products of animal slaughter such as animal-derived rennet, found in some cheeses, and gelatin. Vegetarians may consume these or other unfamiliar animal ingredients unknowingly, however.

Vegetarianism may be adopted for ethical, health, environmental, religious, political, cultural, aesthetic, economic, or other reasons, and there are a number of vegetarian diets. A lacto-vegetarian diet includes dairy products but not eggs, an ovo-vegetarian diet includes eggs but not dairy products, and an ovo-lacto vegetarian diet includes both eggs and dairy products. A vegan diet excludes all animal products, such as dairy products, eggs, and usually honey. Many vegans also seek to avoid using any other animal-derived products, such as clothing and cosmetics.

Semi-vegetarian diets consist largely of vegetarian foods, but may include fish or poultry, or other meats on an infrequent basis. A pescetarian diet, for example, includes fish but no meat. Those with diets containing fish or poultry may define "meat" as mammalian flesh and may identify with vegetarianism. However, vegetarian groups such as the Vegetarian Society have stated that diets containing fish and poultry are not vegetarian, because fish and birds are animals.