I have become somewhat fascinated with economics and I have been reading some books in my spare time.
I recently read Fiat Currency Inflation in France and in this book inflation means something along the lines of "the expansion of the money supply".
However on wikipedia, the definition is completely different. The source is from a recent college textbook, written by the guy who used to be in charge of the printing press, ben bernanke.
When was the definition changed and why?
This is more an economics question than a historical one, mostly because those definitions are equivalent, albeit in a roundabout way. The equation of exchange relates inflation to the money supply. It says that MV = PY, where M is the quantity of money, V the velocity of money (how often it changes hands), P is the price level, and Y is the GDP. Fischer, who developed this theory in the 1900s, postulated that the velocity of money was constant in the short-term. Converting this equation to a rate equation and eliminating the hypothetical change in V (if it's constant, it doesn't change), you get inflation rate = % change in money supply - % change in GDP. So if all else is equal, an increase in the money supply leads to inflation.
Incidentally, the direct money supply-inflation relationship isn't exact in practice, it does a decent job of predicting inflation. In real life, all else is not equal, but it's not a bad approximation.
And this connection is useful historically--apparently Aristotle noted the connection between the supply of money and prices, and an influx of gold to Spain from the New World caused a drastic increase in price.
Source: "Money, Banking, and the Financial System", R. Glenn Hubbard and Anthony Patrick O'Brien. The latter was a professor of mine on the subject, incidentally.