"Energy subsidy," as the phrase is tossed around Washington, typically refers to any financial help the government gives to producers of oil, wind, or other sectors of the energy industry.
But there's another way to consider energy subsidies that takes a bigger picture and conceives of all manner of help—financial or otherwise—as a subsidy. In that context, letting companies pollute for free, when that pollution carries a real social cost, can be thought of as a subsidy.