It is not my definition; feel free to Google it. The actual reality of bitcoins model falls well within that of a Ponzi scheme as the only derived long-term value comes from new investors and the long term model will continuously bleed more resources than sustainable running the network.
The US dollar is differentiated as it’s a secured currency backed by a legitimate business interest (the US government; citizens have no say in it. It does not matter if they wish to use Euros. They are forced to transact via the currency legally and thus it’s value is secured.)