store detective vs PI

store detective and PI both are nouns.

Word	Noun	Adjective	Verb	Adverb
store detective	Yes	No	No	No
PI	Yes	No	No	No

As nouns, PI is a hypernym of store detective; that is, PI is a word with a broader meaning than store detective:

store detective: a private detective employed by a merchant to stop pilferage
PI: someone who can be employed as a detective to collect information and assist in criminal investigations

Other hypernyms of store detective include operative, private detective, private eye, private investigator, shamus, sherlock.

store detective (noun)	PI (noun)
a private detective employed by a merchant to stop pilferage	an antiviral drug used against HIV; interrupts HIV replication by binding and blocking HIV protease; often used in combination with other drugs
	someone who can be employed as a detective to collect information and assist in criminal investigations
	the scientist in charge of an experiment or research project