If these really are synonyms, then doesn't signaling win by having fewer syllables?

Whoops, I was unclear. When I said it seems like "it's usually used to mean exactly the same thing", I meant that some people on Less Wrong use it that way, not that that's the correct usage.

Using it as a synonym of emotion would indeed be wrong, but I haven't noticed people using it that way. Got a concrete example?

I suppose the only example that comes to mind is the term "affective death spiral", which doesn't seem (to me) to have anything in particular to do with affect rather than emotion. I'm tempted to declare "affect heuristic" an abuse of the term as well, except that "affect heuristic" is a term that's actually used by experts.

I also disagree with the definition you gave, though; it fails to capture what I see as the defining quality of affect that distinguishes it from emotions in general, which is that it has only one axis which runs from good to bad.

Is that your best guess based on seeing the term used many times, or do you have some other type of evidence? That "defining quality" doesn't seem to agree with what Wikipedia says at all.