means A computer system you dialed into with a modem to read messages, play games, and download files—the proto-social network that ran on phone lines and teenage bedrooms.

from Bulletin Board System, coined in 1978 when Ward Christensen and Randy Suess built the first one after a blizzard left them trapped. They wanted to share information without leaving the house. The name borrowed from physical bulletin boards you'd pin notices to, because metaphor was all anyone had.