It may be one of the most familiar words in economics. Inflation has plunged countries into long periods of instability. Central bankers often aspire to be known as “inflation hawks.” Politicians have won elections with promises to combat inflation, only to lose power after failing to do so. Inflation was even declared Public Enemy No. 1 in the United States — by President Gerald Ford in 1974. What, then, is inflation and why is it so important?

Inflation is the rate of increase in prices over a given period of time. Inflation is typically a broad measure, such as the overall increase in prices or the increase in the cost of living in a country. But it can also be more narrowly calculated — for example, for certain goods, such as food, or for services, such as school tuition. Whatever the context, inflation represents how much more expensive the relevant set of goods and/or services has become over a certain period, most commonly a year.

Consumers’ cost of living depends on the prices of the many goods and services they consume and the share of each good or service in the household budget. To measure the average consumer’s cost of living, government agencies conduct household surveys to identify a basket of commonly purchased items and then track the cost of purchasing this basket over time. (Housing expenses, including rent and mortgages, constitute the largest component of the consumer basket in the United States. In other countries, especially poorer ones, food can be biggest part of household budgets.) The cost of this basket at a given time expressed relative to a base year is the consumer price index (CPI) and the percentage change in the CPI over a certain period is consumer price inflation, the most widely used measure of inflation. (For example, if the base year CPI is 100 and the current CPI is 110, inflation is 10 percent over the period.)

There are other important measures of price stability. Core consumer inflation — which excludes prices set by the government and the more volatile prices of products, such as food and energy, that are most affected by seasonal factors or temporary supply conditions — focuses on the underlying and persistent trends in inflation and is also watched closely by policymakers. The overall inflation rate for not just for consumption goods but all goods produced in an economy can be calculated by using the gross domestic product (GDP) deflator, an index with much broader coverage than the CPI.

The CPI basket is mostly kept constant over time for consistency, but is tweaked occasionally to reflect changing consumption patterns — for example, to include new hi-tech goods and to replace items no longer widely purchased. Conversely, the contents of the GDP deflator vary each year by definition because it tracks the prices of everything produced in an economy. This makes the GDP deflator more “current” than the mostly fixed CPI basket, but at the same time, the deflator includes non-consumer items (such as military spending) and is therefore not a good measure of the cost of living.

To the extent that households’ nominal income, which they receive in current money, does not increase as much as prices, they are worse off, because they can afford to purchase less. In other words, their purchasing power or real — inflation-adjusted — income falls. Real income is a proxy for the standard of living. When real incomes are rising, so is the standard of living, and vice versa.

In reality, prices change at different paces. Some, such as the prices of traded commodities, change every day; others, such as wages established by contracts, take longer to adjust (or are “sticky,” in economic parlance). In an inflationary environment, unevenly rising prices inevitably reduce the purchasing power of some consumers and this erosion of real income is the single biggest cost of inflation.

Inflation can also distort purchasing power over time for recipients and payers of fixed interest rates. Take pensioners who receive a fixed 5 percent yearly increase to their pension. If inflation is higher than 5 percent, a pensioner’s purchasing power falls. On the other hand, a borrower who pays a fixed-rate mortgage of 5 percent would benefit from 5 percent inflation, because the real interest rate (the nominal rate minus the inflation rate) would be zero; servicing this debt would be even easier if inflation were higher, as long as the borrower’s income keeps up with inflation. The lender’s real income, of course, suffers. To the extent that inflation is not factored into nominal interest rates, some gain and some lose purchasing power.

Indeed, many countries have grappled with high inflation — and in some cases hyperinflation, 1,000 percent or higher inflation a year. In 2008, Zimbabwe experienced one of the worst cases of hyperinflation ever, with estimated annual inflation at one point of 500 billion percent. Such high levels of inflation have been disastrous and countries have had to take difficult and painful policy measures to bring inflation back to reasonable levels, sometimes by giving up their national currency, as Zimbabwe has.

If rapidly rising prices are bad for the economy, is the opposite, or falling prices, good? It turns out that deflation is not desirable either. When prices are falling, consumers delay making purchases if they can, anticipating lower prices in the future. For the economy this means less economic activity, less income generated by producers and lower economic growth. Japan is one country with a long period of nearly no economic growth largely because of deflation.