Any eating pattern that excludes animal-derived ingredients such as meat. Use a more specific tag if the diet is known to have additional restrictions (eg. eggs, dairy, fruit, raw).
Any habitual eating pattern that avoids the consumption of meat (animal muscle and fat). Questions are relevant when they ask about definitions or other guidance for specific diet patterns, or about effects of diet patterns on individuals and the world around us.
Questions about a specific diet pattern should use the tag corresponding to that diet.
There are many more possible diet patterns (eg. whole food plant-based, high-carb low-fat, junk food vegan) that can be discussed, but haven't earned tags yet because there are too few questions on this site.
If it is not possible to apply a more specific diet tag either because the tag hasn't been created yet, or because the specific restrictions are not known or irrelevant, then it's time to use this tag.
Avoid using this tag when asking questions about a specific food. For example, questions about eating tofu would be relevant to both vegetarians and non-vegetarians. Tofu is not the exclusive property of vegetarians. Questions about specific foods should use the food tag, or one of the more specific tags suggested on the food tag wiki.