What does the letter "S" in the PBS network stand for?

The Public Broadcasting Service (PBS) is an American public broadcaster and non-commercial, free-to-air television network based in Arlington, Virginia. PBS is a publicly funded nonprofit organization and the most prominent provider of educational programmes to public television stations in the United States, distributing shows such as Frontline, Nova, PBS NewsHour, Arthur, Sesame Street, and This Old House.

Source: Wikipedia