howdy vs greeting

howdy and greeting both are nouns.

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

As nouns, greeting is a hypernym of howdy; that is, greeting is a word with a broader meaning than howdy:

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

Other hypernyms of howdy include salutation.

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