How can you describe fossil fuels?

I found the two answers in a dictonary.

1. fossil fuel

noun Energy .

any combustible organic material, as oil, coal, or natural gas, derived from the remains of former life.

2. fossil fuel

A hydrocarbon deposit, such as petroleum, coal, or natural gas, derived from the accumulated remains of ancient plants and animals and used as fuel. carbon dioxide and other greenhouse gases generated by burning fossil fuels are considered to be one of the principal causes of global warming.

I know the question is how do I describe it, but I couldn't think of anything and didn't want to leave this blank.