raider

noun

1. a person or thing that raids.
2. a commando, ranger, or the like, specially trained to participate in military raids.
3. a light, fast warship, aircraft, etc., used in such a raid.
4. a person who seizes control of a company, as by secretly buying stock and gathering proxies.
5. Informal. a person who works within an organization for the purpose of gathering evidence of wrongdoing.