greeting vs hullo

greeting and hullo both are nouns.

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

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

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

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