The common abbreviation for Music EDitor, a very popular sample sequencer originally developed by Teijo Kinnunen for the Amiga computers. The original version featured four output channels, and supported samples of 8-11 KHz output frequency.
Sample sequencers were popular due to the small (predominately floppy) disks used on the early Amiga computers, because the two other options for making computer music at the time had their own associated problems. One option was chip music (similar to what one would do on a Commodore 64) which took up virtually no space, but didn't offer anything that sounded like real musical instruments, the other was storing tunes as one big sample (similar to current WAVs and MP3s), which took up WAY too much space. A sample sequencer allowed the clever computer composer to load several small samples of instrument sounds, and store a programmed sequence of alternating pitches and rhythms of those samples, thus (in theory) getting the best of both worlds.
MED was a competitor to the popular Tracker sequencers, notably SoundTracker and ProTracker. The Tracker programs output .MOD (music MODule) files, whereas MED had its own format, .MED -- it was, however, capable of outputting .MODs as well, for compatibility.
A later incarnation of MED was OctaMED, which was the Amiga's first eight-channel sequencer. This was quite a hacking feat on Kinnunen's part, since the Amiga only supported four channels in hardware. Both MED and OctaMED also allowed output to external audio equipment if the Amiga had a MIDI Interface, thus allowing for a much wider range of audio capabilities.
MED was very popular with demo scene crews and game houses on the Amiga platform. After Commodore went bankrupt and the Amiga switched owners too rapidly for even the most hardcore Amigan to keep track of, MED faded into obscurity with the platform. In 1998, Kinnunen handed over the project to programmer Andy Philpott, who built a complete reincarnation of MED, named MED SoundStudio, which runs on the Windows and BeOS platforms.