informer
n.n. a person who tells the police or the government about a crime or a secret. They usually do this to get a reward or to stay out of trouble themselves.
n. a person who provides information to the police or a government agency, typically in exchange for a reward or protection. Often carries a negative connotation of betrayal or lack of loyalty.
The police are looking for an informer who leaked the secret.
The witness was identified as an informer who had previously worked for the rival gang.
The investigation into the corruption scandal revealed that an informer within the department had provided the evidence necessary to secure the convictions of several high-ranking officials.
Inherited from Middle English enformour, from Anglo-Norman enfourmour; equivalent to inform + -er.