I see all sorts of words being used for stuff like this and it got me wondering whether there is an agreed-upon proper naming for all this or if it's just a use-what-you-feel-like-at-the-moment thing.

Umm, built-in I guess.

Unmodded Morrowind is usually referred to as vanilla Morrowind.

"Factory" short for factory model/factory standard.

"vanilla"?

Vanilla.

I say stock, but I picked that up from a community for a different game.

Chocolate.