Market capitalization is simply the calculation of the projected total value of all units of a certain asset that is traded in a marketplace based on its current price. So for example, if you have a stock company that issued 1000 shares in total and 1 share recently traded for $ 500 in the marketplace, the market capitalization of said company is $500 X 1000 = $ 500.000.
Bitcoin has currently 16 million Bitcoin mined and one Bitcoin currently trades for roughly $ 750. So Bitcoin's market capitalization measured in dollar is $ 750 X 16 million = $ 12 billion. Bitcoin's market capitalization measured in Bitcoin is however 16 million - quite logically...
Market capitalization is a projected, theoretical value. It's most likely not the amount of money you will receive when liquidating all shares of a stock / Bitcoin in the open market, because the price will drop because of it.
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