Reducing the discussion to a dictionary definition is emblematic of many of the problems in today's political discourse. This pithy, literal definition of a term is not relevant, especially when that same term, in the same context, is discussed at length, in light of various philosophies on the matter, in the Federalist Papers.
I don't think there's a better source of information about whether the government of the united States of America is a democracy than the position papers written to advance the four corners of its founding instruments, and they deliver an unambiguous "no."
Let's say, for a a moment, that your definition, which seems to conflate these two distinguishable scenarios (one in which a government's power is strictly limited and enumerated, and another in which that power extends to the bounds of whatever a voting majority or their representatives might desire), is a reasonable one.
How, then, do you discuss this distinction?
...and even by your definition, while an individual state might quality, the government of the USA does not: "supreme power" is wholly unavailable through the US government, at least in its legitimate purview (of course it exceeds this boundary frequently and flagrantly). It has dominion only over the specific areas outlined in the constitution; those not mentioned are reserved for the states or the people.
