What does the "S" stand for in VHS?

VHS (Video Home System) is a discontinued standard for consumer-level analog video recording on tape cassettes, introduced in 1976 by the Victor Company of Japan (JVC). It was the dominant home video format throughout the tape media period throughout the 1980s and 1990s. The television industry adopted magnetic tape video recording in the 1950s in the form of the first commercialized video tape recorders (VTRs), but the devices were expensive and used only in professional environments.

Source: Wikipedia