Heres a good source I just found as so many were looking for sources here.

I believe it was around the 1960s or 1970s when most economists started using "inflation" to mean price increases.

The history there is pretty fascinating, it was basically a reaction by modern monetary theorists who really had to redefine it for their economic system to make sense. A core goal in MMT is to have a fiat currency and controls in place to let you manipulate the money supply quickly in an attempt to move the economy in one direction or another. With the original definition, inflation is actually the tool used by MMT rather than an indicator of economic health.

Since most of the people arguing about the term care about a specific class of effect, the term grew to encompass that type of effect. As our understanding of the cause of that effect grew, the term shifted to primarily meaning the effect.

This all makes complete sense since most people don't care about the cause in itself but about how prices are changing.

I would hope that isn't true, an economy would function horribly if we only cared about the price change percentage and didn't care why it happened. If prices went up because most people had more money to spend you should act much differently than if prices went up because supply collapsed, for example.

That isn't what I said.

The causes of inflation to matter, but we generally only care about them because they cause inflation. We don't tend to care nearly as much about the causes in and of themselves.

Thus as the argument about how much inflation there should be progressed, it is perfectly natural that the term came to refer to the part of the debate we actually care (how fast prices rise) about rather than factor that can sometimes cause it.