In recent weeks, President Trump has been talking about a substance he says is key to ending the United States’ war against Iran: “nuclear dust.”

In the president’s telling, Iran’s nuclear program was so badly damaged by U.S. bombs last year that all that remains under the rubble is a sort of powdery aftermath.

The phrase “nuclear dust” seemed designed to diminish the importance of what Mr. Trump is actually talking about — Iran’s stockpile of near-bomb-grade uranium, which is stored in canisters about the size of large scuba tanks.

The material is not, in fact, “dust.” It is typically a gas when stored inside the canisters, though it becomes a solid at room temperature. It is a volatile and highly toxic substance if it comes into contact with moisture and, if mishandled, can trigger a nuclear reaction.

Mr. Trump’s phrase oversimplifies the complex tasks of enriching uranium, to say nothing of negotiating an end to the war. It’s also a phrase nuclear experts say they’ve never heard before.

“I just interpreted it as Trump’s kind of colorful way of talking,” said Matthew Kroenig, the senior director of the Atlantic Council’s Scowcroft Center for Strategy and Security.

Here’s a closer look at what Mr. Trump means when he talks about “nuclear dust,” and why it’s important for an end to the conflict.

What is ‘nuclear dust’?

Mr. Trump is referring chiefly to the uranium Iran has enriched to 60 percent, near the 90 percent purity normally used to make a bomb. There is no use for fuel enriched to that level for, say, producing nuclear power. So it is a warning sign to the international community that Iran could quickly convert the fuel to bomb-grade, even though there would still be many steps to then build a nuclear bomb.

The United States struck three key nuclear sites in June, including a complex outside Isfahan, where much of the near-bomb grade material was believed to be stored.

“It’s not yet bomb-grade, but it’s on the way there, and it was being stored on the nuclear facility at Isfahan,” Mr. Kroenig said. “And so when Isfahan was bombed, that material was presumably entombed there.”

American intelligence officials believe that the Iranians dug down to gain access to the material, though there is no evidence any of it has been moved.

Uranium contains a rare radioactive isotope, called U-235, that can be used to power nuclear reactors at low enrichment levels and to fuel nuclear bombs at much higher levels. The goal of uranium enrichment is to raise the percentage levels of U-235, which is often done by running it through gas centrifuges, machines that spin at supersonic speeds to increase the purity of the fuel.

Why is it important to ending the war?

Mr. Trump has said that Iran had agreed to turn over its nuclear materials to the United States, though Tehran has denied that claim.

“The U.S.A. will get all nuclear dust,” Mr. Trump told a crowd in Arizona last week. “You know what the nuclear dust is? That was that white powdery substance created by our B-2 bombers.”

Iranian enrichment levels have been rising since Mr. Trump withdrew the United States from the Obama-era nuclear deal, known as the Joint Comprehensive Plan of Action, or J.C.P.O.A., saying the agreement wasn’t tough enough.

Mr. Trump then imposed several rounds of American sanctions on Iran. In response, Tehran repeatedly moved beyond the strict limits that the agreement had placed on its uranium enrichment, and began to resume production of nuclear material.

“They were enriching at very low levels before Trump administration withdrew the United States from the J.C.P.O.A.,” said Justin Logan, the director of defense and foreign policy studies at the Cato Institute, a libertarian-leaning think tank. “So what he is calling ‘nuclear dust’ did not exist inside Iran after the signing or the first several months of the J.C.P.O.A.”

Can the material be removed during wartime?

Mr. Trump acknowledges removing Iran’s enriched uranium would be difficult. On Truth Social, he said this week that “digging it out will be a long and difficult process.”

It could be almost impossible without Iranian agreement.

“This would be a mission that would take a lot of time, and there would be a lot of nerds that aren’t good at killing people that would need to be involved here,” Mr. Logan said. “So the idea of doing this while we have our swords drawn strikes me as crazy.”

He said it would be similarly difficult for the Iranians to extract the material during the war.

“Trump is correct to say that we have eyes over the target pretty much all the time, and the Iranians couldn’t just swoop in the middle of the night and spirit it out; it’s an extremely volatile substance,” he said. “We don’t know the conditions of the underground storage. Those tanks in which it has been stored might not be in great condition. It’s going to require a lot of nerds on the ground. And that’s true for the Iranians as much as it is true for us.”