Pirate decryption most often refers to the reception of compromised pay TV or pay radio signals without authorization from the original broadcaster. The term "pirate" in this case is used in the sense of copyright infringement and has little or nothing to do with sea piracy or pirate radio, which involved the operation of a small broadcast radio station without lawfully obtaining a license to transmit. The MPAA and other organizations which try to protect copyright and licensing agreements often call such decryption "signal theft" even though there is no direct tangible loss.

The concept of pay TV is almost as old as TV itself and involves a broadcaster deliberately transmitting signals in a non-standard, scrambled or encrypted format in order to charge viewers a subscription fee for the use of a special decoder needed to receive the scrambled broadcast signal.

Early pay TV broadcasts in countries such as the United States used standard over-the-air transmitters; many restrictions applied as anti-siphoning laws were enacted to prevent broadcasters of scrambled signals from engaging in activities to harm the development of standard free-to-air commercial broadcasting. Scrambled signals were limited to large communities which already had a certain minimum number of unencrypted broadcast stations, relegated to certain frequencies. Restrictions were placed on access of pay TV broadcasters to content such as recent feature films in order to give free TV broadcasters a chance to air these programs before they were siphoned away by pay channels.