house detective vs PI

house detective and PI both are nouns.

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

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

house detective: a private detective employed by a hotel or retail store
PI: someone who can be employed as a detective to collect information and assist in criminal investigations

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

house detective (noun)	PI (noun)
a private detective employed by a hotel or retail store	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