vegetarianism vs diet

vegetarianism and diet both are nouns.

Word	Noun	Adjective	Verb	Adverb
vegetarianism	Yes	No	No	No
diet	Yes	No	Yes	No

As nouns, diet is a hypernym of vegetarianism; that is, diet is a word with a broader meaning than vegetarianism:

vegetarianism (noun)	diet (noun)
a diet excluding all meat and fish	the act of restricting your food intake (or your intake of particular foods)
	a prescribed selection of foods
	the usual food and drink consumed by an organism (person or animal)
	a legislative assembly in certain countries (e.g., Japan)

vegetarianism (verb)	diet (verb)
	eat sparingly, for health reasons or to lose weight
	follow a regimen or a diet, as for health reasons