Synthestration is the art of composing music in the form of a MIDI mockup. A mockup is an extensive demo of a musical recording, for playback by computers triggering virtual instrument software or hardware, to emulate an orchestral recording.
The process involves inputting notes into a computer sequencing program as MIDI data, which then produces those tones via virtual instrument software/hardware. This is followed by significant programming in order to realistically mimic the ensemble playing of an orchestra.
The term is a combination of the words 'synth(esizer)' and 'orchestration'.