From Black's Law Dictionary:

Ponzi Scheme: "...Money from the new investors is used directly to repay or pay interest to earlier investors"

Bitcoin does not fit the definition of Ponzi scheme for numerous reasons:

- There are no paid dividends to anyone

- The purpose of using bitcoin isn’t to recruit new participants

- There’s no centralized body that funnels money up to the top

- Unlike Ponzi schemes, Bitcoin retains value and continues to function even if no new participants join the ecosystem

Bitcoin fails the litmus test for a Ponzi scheme in any number of ways. The boring reality is that it is just a value transfer network that grows more useful the more people join it (much like a telephone network grows more valuable the more people are reachable on it).