hullo vs salutation

hullo and salutation both are nouns.

Word	Noun	Adjective	Verb	Adverb
hullo	Yes	No	No	No
salutation	Yes	No	No	No

As nouns, salutation is a hypernym of hullo; that is, salutation is a word with a broader meaning than hullo:

hullo: an expression of greeting
salutation: (usually plural) an acknowledgment or expression of good will (especially on meeting)

Other hypernyms of hullo include greeting.

hullo (noun)	salutation (noun)
an expression of greeting	(usually plural) an acknowledgment or expression of good will (especially on meeting)
	an act of honor or courteous recognition
	word of greeting used to begin a letter