essay vs theme

essay and theme both are nouns.

Word	Noun	Adjective	Verb	Adverb
essay	Yes	No	Yes	No
theme	Yes	No	Yes	No

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

Other hyponyms of essay include composition, paper, report, disquisition, memoir, thanatopsis.

essay (noun)	theme (noun)
a tentative attempt	a unifying idea that is a recurrent element in literary or artistic work
an analytic or interpretive literary composition	the subject matter of a conversation or discussion
	an essay (especially one written as an assignment)
	(music) melodic subject of a musical composition
	(linguistics) the form of a word after all affixes are removed

essay (verb)	theme (verb)
make an effort or attempt	provide with a particular theme or motive
put to the test, as for its quality, or give experimental use to