XSMP
X11's protocol for saving a desktop session and bringing it back with every program and window where you left it.
The X Session Management Protocol arrived with X11R6 in 1994. A session manager — xsm in the reference release — tracks every connected client and hands each a unique client ID. At logout it sends SaveYourself: programs save their state and reply with the command needed to restart them, ID included, so they can find that state again next time.
It doesn't run over the X connection but over ICE, Inter-Client Exchange, a separate transport introduced in the same release. The libSM library became the basis of session handling in both GNOME and KDE. In practice support was patchy — plenty of programs answered politely and saved nothing — and Wayland has no widely adopted equivalent.