Confusingly, administration officials have sometimes used “clean coal” to refer to highly efficient coal plants that don’t use carbon capture, but emit somewhat less carbon dioxide into the atmosphere than older coal plants. There is one such highly efficient coal plant currently in operation in Arkansas, but given their high upfront costs, it is unlikely that more will be built soon in the United States.

Coal companies can also take steps to reduce the pollutants generated by burning coal in traditional plants — by, for instance, washing coal that comes out of mines to reduce its ash content. Mr. Trump seemed to be referring to this latter practice in his Phoenix speech as “clean coal.”

Critics note that “clean coal” is a misleading term for any of these techniques. Even a coal power plant that emits fewer pollutants is still a far dirtier way to produce electricity than a natural gas, nuclear, wind or solar plant. In 2014, the Clean Air Task Force estimated that particle pollution from power plants, mainly coal, led to 7,500 premature deaths each year, although that number has been going down over time because of environmental regulations and the retirement of older coal plants in the face of cheap natural gas.

And regardless of plant technology, mining for coal remains a highly polluting practice, often damaging streams and waterways. On Monday, the Trump administration announced that it was canceling a National Academies of Sciences, Engineering and Medicine study into the health impacts from mountaintop removal mining, in which companies in Appalachia blast open the tops of mountains and dump the rubble into nearby valleys.