fast vs diet

fast and diet both are nouns.

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

As nouns, diet is a hyponym of fast; that is, diet is a word with a more specific, narrower meaning than fast:

fast: abstaining from food
diet: the act of restricting your food intake (or your intake of particular foods)

Other hyponyms of fast include dieting, hunger strike, Ramadan.

As verbs, diet is a hyponym of fast; that is, diet is a word with a more specific, narrower meaning than fast:

As verbs, diet is a hyponym of fast; that is, diet is a word with a more specific, narrower meaning than fast:

fast (noun)	diet (noun)
abstaining from food	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)

fast (adjective)	diet (adjective)
acting or moving or capable of acting or moving quickly
at a rapid tempo
(used of timepieces) indicating a time ahead of or later than the correct time
(of a photographic lens or emulsion) causing a shortening of exposure time
unwavering in devotion to friend or vow or cause
securely fixed in place
hurried and brief
unrestrained by convention or morality
resistant to destruction or fading
(of surfaces) conducive to rapid speeds

fast (verb)	diet (verb)
abstain from eating	eat sparingly, for health reasons or to lose weight
abstain from certain foods, as for religious or medical reasons	follow a regimen or a diet, as for health reasons

fast (adverb)	diet (adverb)
quickly or rapidly (often used as a combining form)
firmly or closely