Compare the price of bitcoin now is low than last last year last 2017 there's a big bubble on bitcoin and many people are buying that time. So when bitcoin goes down many people also cry because of there big lost. So now many are holding bitcoin and waiting again for the price bubbles.
What if the future price of Bitcoin goes much higher than its ATH on 2017 and become stable all through out? Would you consider it a bubble? Bitcoin being a bubble or not is subjective depends on how we view it. As long as Bitcoin is gaining value, trading actively and have a huge demand from the market, we will never consider it as Bubble. Price of Bitcoin last 2017 can be considered bubble until it is broken by another Bitcoin price bubble.
Bitcoin :
bit·coin
/ˈbitˌkoin/
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nounTRADEMARK IN UK
a type of digital currency in which a record of transactions is maintained and new units of currency are generated by the computational solution of mathematical problems, and which operates independently of a central bank.
"bitcoin has become a hot commodity among speculators"
Bubble:
Bubble (physics), a globule of one substance in another, usually gas in a liquid
Can't find Bitcoin on that bubble nor bubble on that Bitcoin.

Then why do we keep on saying Bitcoin is a bubble, why not say Bitcoin price is a bubble?