Stool pigeon noun – A person who provides information about another's wrongdoing. Shill and stool pigeon are semantically related in informer to police topic. In some cases you can use "Shill" instead a noun phrase "Stool pigeon".
Shill noun – A decoy who acts as an enthusiastic customer in order to stimulate the participation of others. Stool pigeon and shill are semantically related in bait topic. You can use "Stool pigeon" instead a noun "Shill".