The term "S-PDIF" is an acronym for "Sony-Phillips Digital Interface Format" and is sometimes written as "SPDIF." It describes both the format of the encoding of the digital audio, clocking, and non-audio information, and the physical format (unbalanced 75 Ohm coaxial electrical or optical). Teh most common fomr of electrical connection is an "RCA" (or IEC or "Cinch") connector. The most common optical connection is "Toslink;" which was developed originally by Toshiba.
In order to address the need for a standardized connection to carry stereo linear PCM digital audio between pieces of equipment, SONY/Phillips adopted a version of the professional AES digital audio interface standard for consumer use. As technology progressed, the connection was adapted for alternative formats employing data compression for multi-channel audio formats for video (DTS and AC-3).