I do prefer a fairly liberal definition of "instrumental" that would include basically all electronic music, because the division becomes very hard to define otherwise. One more narrow definition I have heard is that a song must be playable in real-time by people with instruments, where "instrument" does not include a synth, sampler, sequencer or whatver. So that leaves classical, jazz, instrumental rock, etc, but what about instrumental rock that utilizes overdubbing? If that's okay, and effects are okay, what about sampled compositions by Amon Tobin or whoever? Instrumental rock with synths? Nevermind the fact that synths are instruments, except when they're computer-modeled virtual instruments, which make sound, so how is that virtual? It's all so confusing and I get rambly.
Of course, I understand a desire to narrow things down a bit too, lest one get thrown hallucinogen-twisted curveballs like Shpongle. Shpongle are great.
