anion vs hydroxyl ion

anion and hydroxyl ion both are nouns.

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

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

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

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