A fossil fuel subsidy is
Any government action that lowers the cost of fossil fuel energy production
Raises the price received by energy producers
Lowers the price paid by energy consumers.

Essentially, it’s anything that rigs the game in favor of fossil fuels compared to other energy sources.

Then you add the externalities that are not paid. Which effectively is a subsidy.

Then you have the use of US forces in oil rich nations.. that "lowers the price of fossile fuels".. even though its paid by taxpayers paying for military, its a subsidy.