Vegetarianism is the practice of a diet that excludes all animal flesh, including poultry, game, fish, shellfish or crustacea, and slaughter by-products. The reasons for choosing vegetarianism may be related to morality, religion, culture, ethics, aesthetics, environment, society, economy, politics, taste, or health.

There are several variants of the diet, some of which also exclude eggs and/or some products produced from animal labor such as dairy products and honey. Veganism, for example, excludes all animal products from diet. By some strict definitions, animal products are not used for attire either, whether or not the production of clothing or items has directly involved the actual death of an animal (dairy, eggs, honey, wool, silk, down feathers, etc.) A generic term for both vegetarianism and veganism, as well as for similar diets, is "Plant-based diets".

Properly planned vegetarian diets have been found to satisfy the nutritional needs for all stages of life, and large-scale studies have shown vegetarianism to increase longevity, improve health, and significantly lower risks of cancer and other diseases.