An informer is someone who secretly tells authorities about illegal activities or rule-breaking, especially when they're part of the group doing those things. An informer might tell police about a crime their friends are planning, or report classmates who are breaking school rules.

The word carries a complicated feeling. While informing on serious crimes can protect people from harm, informers often face anger and distrust because they betray the confidence of others. During wartime, informers might report enemy spies to protect their country. In organized crime, an informer who tells police about criminal plans is sometimes called a snitch or rat, names that show how other criminals view this betrayal.

History shows both heroic and troubling uses of informers. Some informers have helped stop terrible crimes, while others have falsely accused innocent people to save themselves or gain rewards. In some countries, governments have used networks of informers to control citizens through fear, making people afraid to speak freely, even to friends.

The distinction between an informer and a regular witness matters: a witness honestly reports what they saw, while an informer typically operates in secret, often from inside a group, and their motives might include avoiding punishment, seeking reward money, or genuine concern about preventing harm.