I am a big fan of efforts to define “instrument” in a way that is expansive enough to include consumer music playback devices.
Ethan Hein does it in this blog post, in a way that is totally unlike how I think of it, but still fascinating:
There are a lot of different musical instruments out there. Just about all of them share four basic components: an oscillator, a source of noise, some kind of modulation, and a resonator.
For the curious, my definition has more to do with the relationship between players and machinery that produces predefined groups of frequencies, and is introduced somewhere in my undergrad thesis [pdf link].
(via Ethan Hein’s metablog)