[nextaccountic]

> Like everything in mathematics, "obvious" is a term of art. Broadly speaking, it refers to a fact, proof, consequence, which is necessary for the proof to advance, but which is already established elsewhere, so it does not in itself aid in understanding the proof being presented.

"Trivial" is a term of art (and doesn't mean exactly this) but I'm not sure that "obvious" is.

I think that something already proved is called an immediate corollary if it's a very small step, theorem if it's not that small but well known, a previous result (with a citation!) if not well known, or if you can't cite anything, at least say it's part of "mathematical folklore".