Vegetarisk more often describes food, while the former two describe people and are in my opinion synonyms.

Although (as correctly pointed out by Derpius) the terms "en vegetar" and "en vegetarianer" can be used as synonyms to describe a person, "en vegetarianer" is by far the most commonly used, so I would probably stick to that one. "Vegetar-" can also be used as a prefix to describe something vegetarian, such as "vegetarkost", "vegetarmat", "vegetarrett", "vegetarrestaurant" and so on. These same things can in turn also be described by instead placing one of the words "vegetarisk" or "vegetariansk" in front: "vegetarisk/vegetariansk kost", "vegetarisk/vegetariansk mat", "vegetarisk/vegetariansk rett", "vegetarisk/vegetariansk restaurant" osv.

Vegetarianer = Vegetarian person Vegetarisk = Vegetarian dish/meal etc Vegetar = The broader concept, but can often be used as "vegetarisk".

Grammar-wise you can categorize them as: Vegetar > noun (people or animals with a plant-based diet) Vegetarisk > adjective vegetarianer > noun (a person with a plant-based diet) They can all be used to create compound words: noun+noun > vegetarmat, vegetardyr (herbivores/ vegan animals) noun+noun >vegetarianermat adjective+noun >vegetarisk mat E.g. Vegetardyr som elefanten og flodhesten spiser mye vegetarisk mat.