PI vs inquiry agent

PI and inquiry agent both are nouns.

Word	Noun	Adjective	Verb	Adverb
PI	Yes	No	No	No
inquiry agent	Yes	No	No	No

As nouns, inquiry agent is a hyponym of PI; that is, inquiry agent 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
inquiry agent: a private detective

Other hyponyms of PI include hotel detective, house detective, house dick, store detective.

PI (noun)	inquiry agent (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
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