tub vs hot tub

tub and hot tub both are nouns.

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

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

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

Other hyponyms of tub include footbath, hip bath, sitz bath.

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