PI vs house detective

PI and house detective both are nouns.

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

As nouns, house detective is a hyponym of PI; that is, house 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
house detective: a private detective employed by a hotel or retail store

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

PI (noun)	house 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 hotel or retail store
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