Descriptive of a synth in which every key on the keyboard can be pressed simultaneously, and all will sound a note. A fully polyphonic synth would appear to be the ideal for the performer. However, it is a very expensive design to implement, because of the circuitry required in order to have a full-featured voice architecture. Methods have been tried such as divide-down and top octave division to reduce the circuitry required, but these wind up limiting the timbral capability of the synth, and so they are usually confined to electric organs and string synths. Additionally, a synth with MIDI may be capable of playing a far greater span of notes than the synth's own keyboard covers, so the definition of "fully polyphonic" becomes problematic in that context. Because few types of music actually require that many notes to be played at once, voice allocation designs usually serve the purpose and are more cost effective. Nearly all polyphonic synth designs since the 1980s have been designed around voice allocation.
The Korg PS-3100 series is one of the few examples of a synth that is fully polyphonic and does not rely on divide-down or top octave division techniques.