informerin‧form‧er /ɪnˈfɔːmə $ -ɔːrmər/ noun [countable] 
► spy someone whose job is to find out secret information about another country: · Stalin controlled a network of spies.· The film is basically a spy story.
► agent/secret agent someone who works for a government or police department in order to get secret information about another country or organization: · a secret agent working for MI5· He is the FBI’s best undercover agent (=one who works secretly and pretends to be someone else).
► double agent someone who finds out an enemy country’s secrets for their own country but who also gives secrets to the enemy: · a former CIA double agent who also worked for the KGB
► mole someone who works for an organization while secretly giving information to its enemies: · A mole in the government was leaking information to the press.
► informer someone who secretly tells the police about criminal activities, especially for money: · Acting on information from an informer, the police raided the house.
► espionage the work that spies do: · He is serving a 20-year prison sentence for espionage.
someone who gives information to the police► informer/informant someone who is part of or closely connected with a criminal organization but who secretly tells the police about its activities: · An informer had warned police about the bombing.police informer/informant: · The three men were released on condition that they became police informants.
► grass British informal someone who secretly gives the police information about someone who is responsible for a crime, in return for money, when this seems disloyal: · I wouldn't inform on you -- I'm no grass.
► nark especially British, informal /narc especially American, informal someone who is friendly with criminals and who secretly tells the police about their activities, especially activities involving illegal drugs: · I wouldn't trust that new guy - I think he's a narc.
► stool pigeon especially American a criminal who helps the police to trap other criminals by telling the police about a crime that is going to take place: · How could he ever live with himself after being a stool pigeon?
► source someone who gives information to the police, a newspaper etc, especially someone who does not want their name to be known: · It's the first duty of a journalist to protect his or her sources (=not say who they are).· Our source informed us that there was a possibility of another attack the following week, possibly in the central London area.
nouninformantinformationinformermisinformationdisinformationadjectiveinformative ≠ uninformativeinformed ≠ uninformedverbinformmisinformadverbinformatively