A Code of Silence is when someone, usually with some sort of authority of power, but not always, witnesses or is privy to the knowledge of an illegal or embarrassing act, but puts comradery, or loyalty to the "unit" (which can be police, military, gangs, among other things) above informing the public, or proper authorities.
Code of Silence is a cult-classic 1985 film staring Chuck Norris as a Chicago police officer who takes down a Chicago drug lord. Typical for the genre and the star, but filmed on location in Chicago with a few sub-plots. It featured him with a more than menacing police fighting machine named "Prowler".