A bubble is when reality can not keep up with expectation. Wow, has bitcoin skyrocketed! Mean reversion is the theory suggesting that prices and returns eventually move back toward the mean or average. This mean or average can be the historical average of the price or return, or another relevant average such as the growth in the economy or the average return of an industry(https://www.investopedia.com/terms/m/meanreversion.asp).Take a look at the charts. 
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