fatigue vs eyestrain

fatigue and eyestrain both are nouns.

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

As nouns, eyestrain is a hyponym of fatigue; that is, eyestrain 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
eyestrain: a tiredness of the eyes caused by prolonged close work by a person with an uncorrected vision problem

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

fatigue (noun)	eyestrain (noun)
labor of a nonmilitary kind done by soldiers (cleaning or digging or draining or so on)	a tiredness of the eyes caused by prolonged close work by a person with an uncorrected vision problem
(always used with a modifier) boredom resulting from overexposure to something
temporary loss of strength and energy resulting from hard physical or mental work
used of materials (especially metals) in a weakened state caused by long stress

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