private detective vs private investigator

private detective and private investigator both are nouns.

Word	Noun	Adjective	Verb	Adverb
private detective	Yes	No	No	No
private investigator	Yes	No	No	No

As nouns, private detective and private investigator are synonyms defined as:

private detective and private investigator: someone who can be employed as a detective to collect information and assist in criminal investigations

Other synonyms of private detective include operative, PI, private eye, shamus, sherlock.

private detective (noun)	private investigator (noun)
someone who can be employed as a detective to collect information and assist in criminal investigations	someone who can be employed as a detective to collect information and assist in criminal investigations