There are piano grades in the UK? Bizarre... well, since we seem to be having a pissing contest of qualifications, here are mine. I've played piano for 19 years, been a piano teacher and competitor, and am currently employed by a university music department. Also, I have two degrees in English with instruction in linguistics. None of this means jack about the quality of my opinion, but hopefully it will assure those-who-care that I am not a total dimwit.
Okay. (Deep breath.)
I never read the start of this discussion on another thread, so forgive me if I miss a point. But it looks to me as though the person who started this thread is asking "what defines an instrument," and most of the respondents are answering "what defines a piano." This all comes down to a matter of semantics. No one goes out of their way to say acoustic guitar, unless they are trying to differentiate it from an electric guitar. There's no reason to specifically say "acoustic piano" unless you need to differentiate between it and a synthetic, digital, or electric piano. Otherwise, most people are going to assume that "piano" refers to the sort you took lessons on, with the strings and hammers and whatnot. This is not right nor wrong, but a matter of linguistic preference. As a pianist, I wouldn't call a keyboard or a synthesizer a "piano"... unless I was playing said synthesizer in a band which did not have an acoustic piano, at which point I most certainly would, because we use shorthand in situations like that.
Anything that can make a sound can be used as a musical instrument. Take, for example, PDQ Bach's "Pervertimento for Bagpipes, Bicycle & Balloons". Yeah, it's a joke composition, but it's real, and it's music.
This whole thing seems to largely be a battle of semantics, and I can tell you that there are better things to fuss over. Don't be prescriptive grammarians! Language and its usage is a living, evolving creature, and there is no such thing as right and wrong -- merely typical, atypical, and varying degrees of social acceptability.
