There are many variations of the vegetarian diet: an ovo-vegetarian diet includes eggs and a lacto-vegetarian diet includes dairy products, while a lacto-ovo vegetarian diet includes both. As the strictest of vegetarian diets, a vegan diet excludes all animal products, and can be accompanied by abstention from the use of animal-derived products, such as leather shoes.

The earliest record of vegetarianism comes from the 9th century BCE, inculcating tolerance towards all living beings. Parshwanatha and Mahavira, the 23rd and 24th tirthankaras in Jainism, respectively, revived and advocated ahimsa and Jain vegetarianism between the 8th and 6th centuries BCE; the most comprehensive and strictest form of vegetarianism. In Indian culture, vegetarianism has been closely connected with the attitude of nonviolence towards animals (called ahimsa in India) for millennia and was promoted by religious groups and philosophers. The Ācārāṅga Sūtra from 5th century BCE advocates Jain-vegetarianism; and forbids the monks from walking on grass in order to avoid inflicting pain on them and prevent small insects dwelling inside from getting killed. The ancient Indian work of the Tirukkuṟaḷ, dated before the 5th century CE, explicitly and unambiguously emphasizes shunning meat and non-killing as a common man's virtues.

Among the Hellenes, Egyptians, and others, vegetarianism had medical or ritual purification purposes. Vegetarianism was also practiced in ancient Greece and the earliest reliable evidence for vegetarian theory and practice in Greece dates from the 6th century BCE. The Orphics, a religious movement spreading in Greece at that time, also practiced and promoted vegetarianism. Greek teacher Pythagoras, who promoted the altruistic doctrine of metempsychosis, may have practiced vegetarianism, but is also recorded as eating meat. A fictionalized portrayal of Pythagoras appears in Ovid's Metamorphoses, in which he advocates a form of strict vegetarianism. It was through this portrayal that Pythagoras was best known to English-speakers throughout the early modern period and, prior to the coinage of the word "vegetarianism", vegetarians were referred to in English as "Pythagoreans".

The first written use of the term "vegetarian" originated in the early 19th century, when authors referred to a vegetable regimen diet. Historically, 'vegetable' could be used to refer to any type of edible vegetation. Modern dictionaries explain its origin as a compound of vegetable (adjective) and the suffix -arian (in the sense of agrarian). The term was popularized with the foundation of the Vegetarian Society in Manchester in 1847, although it may have appeared in print before 1847. The earliest occurrences of the term seem to be related to Alcott House—a school on the north side of Ham Common, London—which was opened in July 1838 by James Pierrepont Greaves.

Western vegetarian diets are typically high in carotenoids, but relatively low in omega-3 fatty acids and vitamin B12. Vegans can have particularly low intake of vitamin B and calcium if they do not eat enough items such as collard greens, leafy greens, tempeh and tofu (soy). In contrast, high levels of dietary fibre, folic acid, vitamins C and E, and magnesium, and low consumption of saturated fat are all considered to be beneficial aspects of a vegetarian diet.