> Where else have you seen this

Node.js is easily the most famous example. Also deno, bun, winterjs, and probably a bunch more.

Someone upthread just linked a bunch of other equivalent lua projects that also refer to themselves as runtimes: https://news.ycombinator.com/item?id=46036362

I've also seen discussions where wrapping the servo-browser-engine in a UI layer referred to the UI layer as a runtime, though I think that's a substantially less canonical use of the word than referring to the part of an implementation that takes requests from the interpreter and executes them in the surrounding environment as a runtime.