"Tuvok has been a Starfleet security officer for a very long time. Believe me, he's qualified."
A prison guard, corrections officer, or jailor was a guard officer responsible for supervising inmates who were being held in custody of the law, under the command of a warden. The term "screw" was slang term for a prison guard.
After Tom Paris and Harry Kim discovered themselves in Akritirian maximum security detention facility, Kim was surprised to learn that it was "a prison without guards." Paris then explained that "The other inmates say this place is three hundred meters underground. Maybe the Akritirians figure guards aren't necessary." (VOY: "The Chute")