Reel-to-reel is a type of magnetic TAPE recording technology that was used in the mid-20th century for audio and data storage. It involves the use of a large reel of TAPE that is wound between two spools and moved past a recording head. The TAPE is then rewound to the beginning and played back. The term 'reel-to-reel' comes from the fact that the TAPE is wound from one reel to another. The four-letter word associated with this technology is 'TAPE'.