"subsidies" includes both direct and indirect subsidies.

We can measure direct subsidies by measuring real and effective tax rates.

We can measure indirect subsidies like healthcare costs paid by Medicare with subjective valuations of human life and rough estimates of the value of a person's health and contribution to growth in GDP, and future economic security.

But who has the time for this when we're busy paying to help folks who require disaster relief services from the government and NGOs (neither of which are preventing further escalations in costs)

Problem is that if the word "subsidies" is used exclusively to mean a specific and narrow concept around cash transfers when talking about renewables, but then you use the term to include a much broader set of scenarios when talking about fossil fuels, while directly comparing the derived numbers, then you're misusing the language in order to deceive, regardless of all else. You're communicating that the numbers represent the same calculations for both forms of energy, when you know that they actually don't.

Define the terms however you like, as long as you are careful to not to induce people to believe someone that is untrue.

It’s true that you have to measure the same thing, but you also have to measure a useful thing.

If you compare fossil fuels and renewables based on the number of letters in their names, that’s the same thing for both, but it’s not useful.

Comparing explicit monetary subsidies is like that. If you want to include the implicit ones for renewables as well, you definitely should. But giving up and only comparing direct monetary subsidies gives you a woefully incomplete and misleading picture.

You're arguing that we should be comparing the total cost of the fossil fuel and renewable energy industries. If the discussion is mainly about the direction society should take with regard to energy, then I would agree with you. But there are still contexts in which it is useful to talk strictly about subsidies. For example, when discussing how much an industry depends on direct government action, which is the context of this whole discussion we are having right now. (ie Solar and Wind Power So Cheap They’re Outgrowing Subsidies, in which the my definition of subsidy is the one being used, I'm pretty sure.) Yes, there are time where it's more important to discuss the big picture, but there are also times it's worthwhile to discuss the details and language should retain the detail to do both.

There's another reason this is important and it is with regard to the big picture. A lot of people already know what subsidy means and understand it to mean direct government gifts. When you use a different definition of a commonly understood word, what you are doing is defining it stipulatively. If you don't clarify that you are using a stipulative definition ahead of time, then you are committing a fallacy of equivocation. It doesn't matter if you think that your stipulative definition makes more sense or is more useful, people have to be using words the same way in order to communicate. I'd rather not give climate denialists any more fuel than is absolutely necessary and confusing equivocations are unnecessary. Talking about subsidies when you mean subsidy and talking about costs when you mean cost helps avoid them.