President Donald Trump invoked "clean coal" at Tuesday night's rally in Phoenix, repeating a phrase that's used often by coal industry supporters to put a positive spin on the heavily polluting fuel — but one that remains ambiguous in its definition.

The Republican president took credit for the opening of a coal mine in Pennsylvania, saying, "We've ended the war on beautiful, clean coal, and it's just been announced that a second, brand-new coal mine, where they're going to take out clean coal — meaning, they're taking out coal, they're going to clean it — is opening in the state of Pennsylvan...