You may have heard about the Overton window, and that’s not about to stop. With the political landscape shifting in sometimes startling ways, what was once an obscure idea has gained broader relevance.

But while the term has been bandied about lately, it hasn’t always been by people who know what they’re talking about. And it’s important to get this right. You’ve probably noticed that policies once dismissed out of hand — from “Medicare for all” to a 70 percent top tax rate; from sweeping action on climate change to abolishing Immigration and Customs Enforcement — are being discussed in mainstream circles now. The Overton window is a useful way to understand what’s happening.

Joseph P. Overton introduced the concept in the 1990s as an executive at the Mackinac Center for Public Policy, a conservative think tank in Michigan. He never expected it to gain widespread recognition, said Joseph G. Lehman, president of the Mackinac Center, and it didn’t until after Mr. Overton died in 2003.

Mr. Overton just wanted to explain to potential donors what the point of a think tank was, so he created a brochure with a cardboard slider. The brochure listed the range of possible policies on a single issue, from least to most government intervention. On education — an example the Mackinac Center uses — it might run from zero public investment in education to compulsory indoctrination in government schools. But neither of those extremes is going to happen. Only part of the range is achievable, and when Mr. Overton moved his slider, different policies fell into what he called the window of political possibility.