neap tide vs highwater

neap tide and highwater both are nouns.

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

As nouns, highwater is a hypernym of neap tide; that is, highwater is a word with a broader meaning than neap tide:

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

Other hypernyms of neap tide include high tide, high water.

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