Inflation is the devaluation of the currency. The definition has been muddied for a time now but ultimately inflation refers to the expansion of the money supply.

A quibble only because it’s what I do professionally, but that’s not really how people other than macroeconomists concerned with the money supply talk about inflation. It’s usually used as shorthand for the price change for a given standard of living. The importance of that matters quite a lot when you start talking about relative inflation between goods and services. We can very clearly say that textbooks have had massive price inflation in the last 20 years, and that has nothing to do with the money supply, it’s because they are way more expensive than they used to be relative to other goods. Another reason it matters is comparison across regions. If prices are rising faster in Asheville, NC than NYC because it’s a metro transitioning from being cheap to kind of expensive, it doesn’t make good economic sense to say that currency is being devalued at a faster rate in NC. What is actually happening is that prices are rising faster there, again, relative to the rate of change somewhere else.