I personally would prefer to use the word "theory" to mean "a scientific theory that is, by definition, falsifiable". But it's not a strong preference; I merely think that it helps reduce confusion. As long as we make sure to define what you mean by the word ahead of time, we can use the word "theory" in the vernacular sense, as well.
Regarding moral theories, I have to admit that my understanding of them is somewhat shaky. Still, if moral theories are completely unfalsifiable, then how do we compare them to discover which is better ? And if we can't determine which moral theories are better than others, what's the point in talking about them at all ?
I said earlier that Utilitarianism is more like an algorithm than like a scientific theory; the reason I said that is because Utilitarianism doesn't tell you how to obtain the utility function. However, we can still probably say that, given a utility function, Utilitarianism is better than something like Divine Command -- or can we ? If we can, then we are implicitly looking at the results of the application of both of these theories throughout history, and evaluating them according to some criteria, which looks a lot like falsifiability. If we cannot, then what are those moral theories for ?
I personally would prefer to use the word "theory" to mean "a scientific theory that is, by definition, falsifiable"
So would I. But it's just an ambiguous word in English that means different things in different places. As I take it into the extremely foggy areas that also use the word "theory", I'm going for something like "has explanatory power".
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
If continuing the discussion becomes impractical, that means you win at open threads; a celebratory top-level post on the topic is traditional.