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 Pennsylvania, the second one."

The most literal use of "clean coal" refers to coal that's washed as it leaves the mine to remove impurities such as dirt and rocks, a standard industry procedure. Clean coal also is used to refer to coal burned in modern power plants equipped with costly "scrubbers" to remove air pollutants such as nitrogen oxide or mercury.

In recent years, clean coal has become most closely associated with coal-burning power plants that can capture and remove greenhouse gases such as carbon dioxide, which is considered a prime contributor to global warming. However, only a handful of coal plants worldwide have technology in place to capture and sequester carbon dioxide, including the Petra Nova plant in Texas that came online earlier this year. That plant was partially funded by a Department of Energy program for which Trump has proposed deep cuts.

Some researchers say that even carbon capture won't make coal "clean." That's because the gas that's removed during coal-fired power generation can be pumped underground to increase the amount of crude oil extracted from aging reserves. Critics say the greenhouse gases resulting from increased oil production would offset any gains made at the coal plants.

The Pennsylvania mine Trump spoke about Tuesday night will produce coal for making steel.

It's not "brand-new," as Trump said, but has been idle since 2012.

George Dethlefsen, chief executive of the mine's owner, Corsa Coal Corp., said Wednesday the company recently decided to reopen it, citing a steel industry boom that's driven up prices for metallurgical coal.

He also tipped his hat to Trump, who visited a new Corsa mine in June, saying the coal industry no longer fears being "regulated out of business."