Multiplexor - Telecommunications
A Multiplexor, sometimes known as a MUX, is a device that combines multiple signals into one. Within the telephone system, this usually refers to the ability of the telephone company to multiplex multiple voice lines onto one trunk. The trunk is usally in the form of a T1 or T3 lines, depending on the amount of voice lines that need to be multiplexed.
In fiber networks, a multiplexor is sometimes called a concentrator. It can combine signals either by multiplexing different frequencies of light onto one fiber or by inserting the signals of one fiber channel into the signals of another fiber channel.