screen

GNU screen is the original terminal multiplexer, released in 1987. It pioneered the pattern that tmux later refined: detachable sessions, multiple windows inside one terminal, and survival of dropped connections. Many older servers have only screen available.

screen                   # start a session
screen -S work           # named session
screen -ls               # list sessions
screen -r work           # re-attach
screen -d -r work        # detach (if attached elsewhere) and re-attach

Inside screen, the prefix is Ctrl-a: Ctrl-a c new window, Ctrl-a n next, Ctrl-a p previous, Ctrl-a d detach. tmux is generally preferred on new systems for its cleaner configuration, scripting, and pane-splitting, but screen remains a safe default when you can't install anything else.