hi vs how-do-you-do

hi and how-do-you-do both are nouns.

Word	Noun	Adjective	Verb	Adverb
hi	Yes	No	No	No
how-do-you-do	Yes	No	No	No

As nouns, hi and how-do-you-do are synonyms defined as:

Other synonyms of hi include hello, howdy, hullo.

hi (noun)	how-do-you-do (noun)
an expression of greeting	an expression of greeting
	an awkward situation