tmux is short for "terminal multiplexer." It dramatically expands what can be done with terminal windows. It is highly configurable and scriptable.

tmux is an alternative to an older, similar program named "screen". It was created by Nicholas Marriott. The first version was released in 2007.

One benefit of both tmux and screen is realized when working on remote machines where network communication can fail. Both maintain the state of sessions so the user can pick up where they left off by reconnecting and reattaching to the session.

See my Object Computing (OCI) article tmux.