A persistent tendency for nominal prices to increase. Inflation is measured by the proportional changes over time in some appropriate price index, commonly a consumer price index or a GDP deflator. Cost inflation is started by an increase in some element of costs, for example the oil price explosion of 1973–4. Demand inflation is due to too much aggregate demand. Once started, inflation tends to persist through an inflationary spiral, in which various prices and wage rates rise because others have risen. Hyperinflation is extremely rapid inflation, in which prices increase so fast that money largely loses its convenience as a medium of exchange. See also core inflation; cost inflation; creeping inflation; demand inflation; expected inflation; hyperinflation; imported inflation; menu costs of inflation; repressed inflation; shoe-leather costs of inflation; underlying rate of inflation; unexpected inflation.