slack tide vs tide

slack tide and tide both are nouns.

Word	Noun	Adjective	Verb	Adverb
slack tide	Yes	No	No	No
tide	Yes	No	Yes	No

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

slack tide: the occurrence of relatively still water at the turn of the (low) tide
tide: the periodic rise and fall of the sea level under the gravitational pull of the moon

slack tide (noun)	tide (noun)
the occurrence of relatively still water at the turn of the (low) tide	the periodic rise and fall of the sea level under the gravitational pull of the moon
	something that may increase or decrease (like the tides of the sea)
	there are usually two high and two low tides each day