A private investigator, or PI, is a person who undertakesinvestigations. They often work for attorneys in civil cases or onbehalf of a defense attorney. Many work for insurance companies toresolve claims. Before the advent of no-fault divorce, many privateinvestigators were hired to search out evidence of adultery or otherillegal conduct within marriage to establish grounds for a divorce.Despite the lack of legal necessity for such evidence any more,according to press reports collecting evidence of adultery or other "bad behaviour" by spouses and partners is still one of the most profitable activities investigators undertake.

Definition from en.wikipedia.org/wiki/Private_investigator