The technique of sending several separate signals or streams of data over one shared connection at the same time. Where a multiplexer is the device, multiplexing is what it does — the method itself. It is a noun, and it is also the -ing form of the verb multiplex ('the system multiplexes eight channels onto one fibre'). Two main flavours come up again and again. In time-division multiplexing, each signal gets a very short turn in strict rotation, so fast that all of them appear continuous. In frequency-division multiplexing, each signal is given its own band, so they travel together without touching, the way many radio stations share the air. The reason anyone bothers is money and physics: cable, fibre, and radio spectrum are expensive and limited, so squeezing many conversations into one channel is the whole business model of telecoms. The word has spread beyond hardware — programmers talk about multiplexing network connections, and you will hear people joke about multiplexing their attention across three meetings. That last use is casual and figurative, but it shows the core image clearly: many things sharing one narrow channel by taking turns.