I'm pretty rusty coming back to calculus after a while (and was never hugely solid) but I've hit a wall on something that is probably really obvious. It's from some economics I'm trying to understand: "dx/x" keeps on cropping up. Example:

"We denote relative changes by a tilde: x-tilde = dx/x."

And a differentiation they're using to investigate local stability contains several of these terms e.g. dI/I, d-lambda/lambda etc.

But what does it mean? A small change in x with respect to x? But how does that make any sense?

If a fuller example is needed, I can provide a more detailed write-up. I can't find any answer on the interweb or in my physics course unit notes. I thought it might be linearisation or taylor approximation related, but it doesn't seem to be (though in one version of the problem, linearisation is mentioned.)

Thanks and apologies if this is a really dumb question...