I would say, a bubble exists when the price of an asset is way higher than its "real" value.
We know BCs price, but what is it's "real value"? Hard to say, as you can't really put a price tag to the values I mentioned above. But considering the amount of BCs existing, the amount of people in BCs and said values, it might or might not be in a bubble right now.
I see this point a lot, and I feel like you guys are slightly missing the point here. The point isn't to say that Bitcoin itself is a bubble. The word bubble wouldn't make sense in that context. But that its current value as measured in fiat is highly inflated. I'm not saying it's some experiment that's over-valued and will implode and disappear in the future. Quite the contrary. I'm saying, and I think Peter Schiff is trying to say too, that Bitcoin as a tech has a lot of potential for the future. But it's just that, the future, not the present. Yet in the present the price is incredibly high and climbing. We'd be wise to ask why.
Asking what is a bubble seems redundant. The issue being raised is quite simple. What makes up Bitcoin's current price?
People have all sorts of answers when it comes to fiat and gold. Utility, the law, reserve status of the currency, governments buying gold, etc. But when it comes to Bitcoin it's really quite simple: Speculation.
I don't believe anyone could disagree with that. And if so, I'd love to hear it. Speculation seems to be the number1 determining factor for Bitcoin's price. We haven't seen any widespread adoption that would suggest the price should be around $1,000 yet. And we can't chalk it up to inflation or people fleeing from fiat, because we're not seeing a similar move in other assets. So again we're left with speculation.
And unfortunately, Speculation = Bubble.
Use as a currency rather than just money, i.e. a store of value, is ancillary to the valuation. E.g. gold is pretty much never used as a currency, yet it retains extraordinary value. I like to use the example where you've got 1000 people using it for daily purchases, each storing $100 in bitcoins at any given time, and a single person storing $100,000 of savings in bitcoins. The latter contributes the same amount to the total valuation as the former, yet he's outnumbered by them by three orders of magnitude. Now consider millionaires and billionaires, and you'll quickly see why use as a currency doesn't really matter in terms of valuation (though it matters a ton when it comes to building political capital, and staving off government attacks).
Edit: To give you some numbers: all the gold in the world is worth ~$8T. Bitcoin is currently valued at ~$9B. So the market seems to be saying Bitcoin should supplant 9/8000 ~ 0.1% of gold's role in the economy. The speculators just disagree with this number, and think it should be higher.