Once upon a time, a "mental model" was, in particular, a model: some sort of mental representation of a thing, usually simpler and clearer than the thing itself.
I'll give some concrete-ish examples.
- For anyone you know reasonably well, you can probably make a lot of predictions about how they would react to various things. Whatever you use to do that -- which may well be fuzzy and complicated and largely inaccessible to you -- is your mental model of that person.
- Sometimes you might explicitly model someone as a "homo economicus" money-maximizer, who will always do whatever gets them the most money. This is obviously an extreme simplification, but it has the merit of being something you can explicitly reason about.
- Your mental model of the US government might be that the president makes decisions and everyone else does exactly what he says. This would be a hopelessly wrong model, of course.
- If you're good with differential equations, your mental model of a pandemic might be something like the Kermack-McKendrick SIR model, and that might give you useful intuitions for the consequences of (e.g.) implementing social-distancing measures.
- If you're not so good with differential equations, you might adopt a simpler model that just says that the number of infections is an exponential function of time. Even that's enough to make better decisions than many people do.
But here "mental model" seems to be being used to be generalized way beyond that, to include any way of thinking or broadly applicable idea. A few examples from the Farnam Street list linked near the start of the article: "First principles thinking", "Thought experiment", "Second-order thinking", "Inversion", "Occam's Razor". Some of these are useful tools when building mental models. Some of them are useful things to do, or keep in mind, whatever sort of thinking you're doing. None of them is a model.
In fact, almost nothing in the Farnam Street list is a model. But some of the things in the list are useful metaphors, which you could consider to be tiny models, or model-parts. (For instance: "velocity". Velocity is not a mental model, but you might often want to build mental models that include something you could reasonably call "velocity".)
Is the usage of "mental model" to mean "any thinking tool at all" actually widespread? I've seen it before, but I think the other instances I've seen have been closely linked with this one -- all, I think, quoting Charlie Munger. (I don't know whether Munger's own use of the term is as broad as e.g. Farnam Street's.)
I hope it isn't; the narrower notion in which a mental model is actually a model seems to me a valuable one, and it seems easier to find other terms that convey the broader idea (e.g., "thinking tool", "general principle", "idea") than good replacements for the narrower one.