Tmux is a software application that can be used to multiplex

Multiplex

-Science and engineering:* Multiplexing, combining many signals into a single transmission circuit or channel* Multiplexer, an electronic device that accomplishes multiplexing* Multiplex , a laboratory procedure in molecular biology-Entertainment:...

 several virtual console

Virtual console

A virtual console – also known as a virtual terminal – is a conceptual combination of the keyboard and display for a computer user interface. It is a feature of some operating systems such as UnixWare, Linux, and BSD, in which the system console of the computer can be used to switch between...

s, allowing a user to access multiple separate terminal sessions inside a single terminal window or remote terminal session. It is useful for dealing with multiple programs from a command line interface, and for separating programs from the Unix shell

Unix shell

A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems...

 that started the program.

It's considered as an alternative to the well-known software GNU Screen

GNU Screen

GNU Screen is a software application that can be used to multiplex several virtual consoles, allowing a user to access multiple separate terminal sessions inside a single terminal window or remote terminal session...

.