Informant is a term in used in Forensic psychology that refers to a person who provides law enforcement with information in return for money or other motivations.

Other /More definition:
Informant refers to a person who is well versed in the social phenomenon that you wish to study and who is willing to tell you what he or she knows about it. If you were planning participant observation among the members of a religious sect, you would do well to make friends with someone who already knows about them, possibly a member of the sect who could give you some background information about them. The word Informant should not be confused with a word Respondent.