A "bubble" imo is a positive price movement when there is nothing to back it up anymore.
You can get into a cascading effect where the price is driven sky-high because people are investing in it because the prices are going sky-high turning the whole thing into a self-fulfilling prophecy.
Only when a person or a group, which have a big enough share, figure out that the price is utter bullshit and pull out (or no new investors want in) does the "bubble" "pop". Due to the fact that the price plummets and everyone already knew that the price was bullshit, will everyone start making a run for the exit driving the price into a crash.
With bitcoin it is hard to tell if we are in a "bubble". The price is based on a tekst string and has no usable value, this is equal to fiat where the paper itself has no real value. Is the euro a bubble, is the dollar a bubble? these questions are equally hard to answer. If fiat can survive bitcoin most certainly will survive since nobody controls it. Some virtual coins will therefore become strong like euro's or dollars in fiat-land and others will become virtual Zimbabwian dollars which are completely worthless.
It's all about trust and acceptance and only time will tell which will survive. As we know from historic events like VHS vs Betamax we know that it is not always the best that wins.
Are we in a bubble? Nobody knows. Does it feel like a bubble? Not to me but who am I.
A bubble is something which crashes in price subsequently. If Btc goes up in price due to more widespread adoption then it is not a bubble. If btcs fall in value due to regulations or protocol problems then it is a bubble.
Personally, I don't think it is a bubble because Btc has not even penetrated 1% of the human population. Also, it's proven pretty difficult to kill the Btc protocol and the blockchain provides an alternate way for people to transact if they don't want third party intervention.
To my mind, another reason why btcs are probably not a bubble is the almost complete absence of direct leverage/credit in driving up Btc prices. The big leveraged bets are yet to be made by speculators. Once the big leveraged bets have been made, it will be possible for Btc prices to fall if the leveraged plays are withdrawn. Until then, IMHO current Btc prices are nowhere near bubble valuations.