sight vs espy

sight is a noun but espy is not a noun.

Word	Noun	Adjective	Verb	Adverb
sight	Yes	No	Yes	No
espy	No	No	Yes	No

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

Other hyponyms of sight include descry, spot, spy, detect, discover, find, notice, observe.

sight (noun)	espy (noun)
the act of looking or seeing or observing
the ability to see; the visual faculty
an instance of visual perception
the range of vision
a range of mental vision
a place of interest, especially to visitors
anything that is seen
(often followed by `of') a large number or amount or extent

sight (verb)	espy (verb)
catch sight of; to perceive with the eyes	catch sight of
take aim by looking through the sights of a gun (or other device)