A "stem" is basically a sub mix or partial mix of only some of the tracks of a song. For example, one might take all of the drum tracks only and mix just those into a stereo submix. This would be called the "drums stem" of that song.
I think you're right on both counts. Al. "Stems", while still meaning pretty much the same thing, has a kind of "richer" meaning in AV post, where you can have the fx/foley stems, the music soundtrack stems, dialog stems, and so forth, where the stems can be broken down by source category.
Personally speaking, and kind of weird when you think about it, the only time I use - or mostly have heard used - the term "stem" in audio-only music production is when talking about what kind of package to send to the mastering engineer. If I were to send him a package of submixes along with the full stereo mixdown, I usually call that "sending the stems". But as long as it stays in the realm of mixing, I just refer to them as "submixes" as you say. A submix doesn't often get referred to as a "stem" until it is sent to an ME. Go figure .