1. A "spook" is a slang term referring to federal agents or spies. Spooks include agents working with the FBI, NSA or other secretive government agencies. It is most frequently used by hackers and conspiracy theorists to refer to those agents who investigate computer crime and/or unexplained phenomena such as ghosts, UFOs and other X-Files-esque subjects. The term itself tends to imply paranoia.
Also: "feds," although this term generally refers to any member of the federal government.