An informer is someone who secretly tells the police, the army, or another official group about people who are breaking the law. Very often they do it for money, or because the police have promised to go easier on them. The word feels cold, even ugly: an informer is not a brave witness in court, but a person who talks behind someone’s back. Though actually, the same person may be called an informant in police talk, which sounds cleaner and more like a job. Friends and family usually say it with anger. You will hear it in crime news, spy stories, and history — for example, someone who told the army where fighters were hiding. It is not used for a person who simply gives you useful facts.