What is the definition of inflation?

I am assuming you are talking about market inflation, which is the rate in which goods and stocks prices rise. As a result of inflation, the purchasing power of a unit of currency falls. For example, if the inflation rate is 2%, then a pack of gum that costs $1 in a given year will cost $1.02 the next year. As goods and services require more money to purchase, the implicit value of that money falls.