highwater vs neap

highwater and neap both are nouns.

Word	Noun	Adjective	Verb	Adverb
highwater	Yes	No	No	No
neap	Yes	No	No	No

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

highwater: the tide when the water is highest
neap: a less than average tide occurring at the first and third quarters of the moon

Other hyponyms of highwater include direct tide, neap tide, springtide.

highwater (noun)	neap (noun)
the tide when the water is highest	a less than average tide occurring at the first and third quarters of the moon