The rate at which the general level of prices for goods and services is rising, and, subsequently, purchasing power is falling.
http://en.wikipedia.org/wiki/Inflation
http://www.investopedia.com/terms/i/inflation.asp
I'm referring to what Wikipedia calls Monetary Inflation which is the original meaning of the term inflation (from the latin word "inflare"). This is in line with the Austrian School of Economics. I refuse to call price increases inflation, because this hides the cause (increase in monetary supply) of the effect (rising prices).
I forgot, I'm on bitcointalk with some old school bitcoiners, where everybody is an Austrian School of Economics follower...