The word "告密者" is formed by combining the character 告, meaning "to tell", with 密, meaning "secret", to create the verb phrase "告密", meaning "to inform on someone" or "to reveal secrets". The character 者, meaning "person" or "one who does", is then attached as a suffix to denote the actor, resulting in the noun "告密者", meaning "informer" or "a person who reveals secrets".