SSRI vs antidepressant drug

SSRI and antidepressant drug both are nouns.

Word	Noun	Adjective	Verb	Adverb
SSRI	Yes	No	No	No
antidepressant drug	Yes	No	No	No

As nouns, antidepressant drug is a hypernym of SSRI; that is, antidepressant drug is a word with a broader meaning than SSRI:

SSRI: an antidepressant drug that acts by blocking the reuptake of serotonin so that more serotonin is available to act on receptors in the brain
antidepressant drug: any of a class of drugs used to treat depression; often have undesirable side effects

Other hypernyms of SSRI include antidepressant.

SSRI (noun)	antidepressant drug (noun)
an antidepressant drug that acts by blocking the reuptake of serotonin so that more serotonin is available to act on receptors in the brain	any of a class of drugs used to treat depression; often have undesirable side effects