hot tub vs tub

hot tub and tub both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot tub	Yes	No	No	No
tub	Yes	No	No	No

As nouns, tub is a hypernym of hot tub; that is, tub is a word with a broader meaning than hot tub:

hot tub: a very large tub (large enough for more than one bather) filled with hot water
tub: a relatively large open container that you fill with water and use to wash the body

Other hypernyms of hot tub include bathing tub, bathtub.

hot tub (noun)	tub (noun)
a very large tub (large enough for more than one bather) filled with hot water	a large open vessel for holding or storing liquids
	a relatively large open container that you fill with water and use to wash the body
	the amount that a tub will hold