I don't like to think that inflation = rising prices because it complicates things, although this is the mainstream idea.
OP is right.
Inflation is an increase in money supply. Deflation is a decrease in the money supply.
Higher prices and inflation are not the same thing. Higher prices are a
consequence of inflation. Inflation is a
cause of higher prices.
Semantics.
But...
You can have both inflation and falling prices (computers e.g.), or inflation with prices staying the same, or inflation with rising prices (most common).
So, according to this logic...
Bitcoin is inflationary until 2140, not deflationary.
Inflation starts when whomever controls the printing presses & mints of a given fiat currency decide to fire up those presses & mints to make a few billion/trillion more for themselves and their pals.
+1 best answer
[...] A rise in the general price index is a symptom of inflation. The cause of inflation, and therefore what inflation actually is, is the expansion of the monetary base in excess of the economic/population/general growth rate. Take your pick.
+1