Literally the middle range of something. In audio we use this term to loosely define the middle range of audio frequencies. It doesn’t have a precise definition in terms of which frequencies are included, and is consequently not used that often in pro audio discussions where specific information is required, but it is acceptable as a way to get across the general idea. It’s not uncommon to find mixers that have a three-band equalizer on each channel, for example. The bands are often labeled low, mid (range), and high and represent long standing paradigms of two and three band EQ circuits on hi-fi systems pro audio equipment.