anion vs hydroxide ion

anion and hydroxide ion both are nouns.

Word	Noun	Adjective	Verb	Adverb
anion	Yes	No	No	No
hydroxide ion	Yes	No	No	No

As nouns, hydroxide ion is a hyponym of anion; that is, hydroxide ion is a word with a more specific, narrower meaning than anion:

Other hyponyms of anion include hydroxyl ion, superoxide, superoxide anion.

anion (noun)	hydroxide ion (noun)
a negatively charged ion	the anion OH having one oxygen and one hydrogen atom