That said, 2 percent is a fairly arbitrary number, and some economists argue that steady inflation at a somewhat higher level would help prevent recessions.

The important thing is arguably less the exact level of inflation and more that it is fairly stable over time. It is big swings that tend to be most disruptive, favoring either debtors (inflation) or creditors (deflation), and generally contributing to lack of faith in a country’s financial system.

For now, though, 2 percent is the goal.

How much inflation are we getting?

Under the inflation measure that the Federal Reserve most focuses on, prices rose only 1.5 percent in 2017, below the target. That’s based on the personal consumption expenditures price index, excluding food and energy (the logic being that commodity prices can swing wildly for reasons unrelated to underlying inflation trends).

That’s not far from the 2 percent target. But the numbers have been undershooting that target continuously since 2012. That undershooting has been a key rationale for the Fed’s keeping interest rates low — its aim is to boost economic growth and thus help get inflation up to the 2 percent target.

Wait, what does inflation have to do with economic growth?

The answer is more uncertain than you might think. A central component of the models that mainstream economists have used for decades is that the inflation rate is shaped by the amount of “slack” or unused capacity in the economy, especially unemployment.

The intuition goes like this: When the unemployment rate is high, there are lots of workers available for any employer that wants to hire them. So employers don’t need to compete for workers by paying higher wages. But if the unemployment rate is low, companies have to pay more to get employees, driving up wages. Higher wages in turn mean more money coursing through the economy chasing finite goods and services, creating inflation.

That basic relationship between unemployment and inflation is known as the Phillips Curve. It did a pretty good job explaining inflation trends from the 1950s to the 1980s, and still forms the underpinnings of how many policymakers think about where inflation comes from: that inflation is essentially evidence that the economy is running too hot, producing goods or services at a level that is not sustainable.