fatigue vs exhaustion

fatigue and exhaustion both are nouns.

Word	Noun	Adjective	Verb	Adverb
fatigue	Yes	No	Yes	No
exhaustion	Yes	No	No	No

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

fatigue: temporary loss of strength and energy resulting from hard physical or mental work
exhaustion: extreme fatigue

Other hyponyms of fatigue include asthenopia, eyestrain, jet lag, grogginess, logginess, loginess, senioritis.

fatigue (noun)	exhaustion (noun)
labor of a nonmilitary kind done by soldiers (cleaning or digging or draining or so on)	the act of exhausting something entirely
(always used with a modifier) boredom resulting from overexposure to something	serious weakening and loss of energy
temporary loss of strength and energy resulting from hard physical or mental work	extreme fatigue
used of materials (especially metals) in a weakened state caused by long stress

fatigue (verb)	exhaustion (verb)
exhaust or get tired through overuse or great strain or stress
lose interest or become bored with something or somebody