In mainstream economics, inflation is a rise in the general level of prices, as measured against some baseline of purchasing power.
The prevailing view in mainstream economics is that inflation is caused by the interaction of the supply of money with output and interest rates. In general, mainstream economists divide into two camps: those who believe that monetary effects dominate all others in setting the rate of inflation, or broadly speaking, monetarists, and those who believe that the interaction of money, interest and output dominate over other effects, or broadly speaking Keynesians.
Related terms include: deflation, a general falling level of prices, disinflation, the reduction of the rate of inflation, hyper-inflation, an out of control inflationary spiral, and reflation, which is an attempt to raise prices to counter act deflationary pressures.
Measures of Inflation
Measuring inflation is a question of econometrics, that is, finding objective ways of comparing nominal prices to real activity. In many places in economics, "real" variables need to be compared, in order to calculate GDP, effective interest rate and improvements in productivity. Each inflationary measure takes a "basket" of good and services, then the prices of the items in the basket are compared to a previous time, then adjustments are made for the changes in the goods in the basket itself. For example if a month ago canned corn was sold in 10 oz. jars, and this month it is sold in 9.5 oz jars, then the prices of the two cans have to be adjusted for the contents. The result is the amount of increase in price which is attributed to "inflation" and not to improvements in productivity.
The role of inflation in the economy
In the long run, inflation is generally believed to be a monetary phenomenon, while in the short and medium term, it is influenced by the relative elasticity of wages, prices and interest rates.  The question of whether the short-term effects last long enough to be important is the central topic of debate between monetarist and Keynesian schools. In monetarism, prices and wages adjust quickly enough to make other factors merely marginal behavior on a general trendline. In the Keynesian view, prices and wages adjust at different rates, and these differences have enough effects on real output to be "long term" in the view of people in an economy.