isomer vs stereoisomer

isomer and stereoisomer both are nouns.

Word	Noun	Adjective	Verb	Adverb
isomer	Yes	No	No	No
stereoisomer	Yes	No	No	No

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

isomer: a compound that exists in forms having different arrangements of atoms but the same molecular weight
stereoisomer: any of a group of isomers in which atoms are linked in the same order but differ in their spatial arrangement.

Other hyponyms of isomer include configurational isomer.

isomer (noun)	stereoisomer (noun)
a compound that exists in forms having different arrangements of atoms but the same molecular weight	any of a group of isomers in which atoms are linked in the same order but differ in their spatial arrangement.