PI vs store detective

PI and store detective both are nouns.

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

As nouns, store detective is a hyponym of PI; that is, store detective is a word with a more specific, narrower meaning than PI:

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

Other hyponyms of PI include hotel detective, house detective, house dick, inquiry agent.

PI (noun)	store detective (noun)
an antiviral drug used against HIV; interrupts HIV replication by binding and blocking HIV protease; often used in combination with other drugs	a private detective employed by a merchant to stop pilferage
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