In a picture where you can only see the top half of a character, like in post #194977, should it be tagged topless or nude, since we're unsure whether or not they're wearing anything down there?

I would say topless. From the image, you can see she's got no shirt/bra on, but you can't see she hasn't got any panties/legwear on.

Agreed, id go with topless tag just because assumptions are not always correct and tagging the visible is the safest bet

Topless, yup.

Alright, thanks.
I was just curious since I've seen both.

Wouldn't this picture appeal to someone who was searching for either tag? Why shouldn't it get both tags?

It (possibly) would, but the way tags have been defined and redefined as of late is trying to maintain the "tag what you see," rather than "who wants to look at this." The redefining of the bdsm tag is the largest example I can think of, where images of nonconsensual bondage that used to be tagged bdsm- because people looking for bdsm would like those pics, too, right?- are no longer, since BDSM is inherently a consensual sort of thing. So no, it should remain only topless. She could be wearing pants. She could have no body below what we see. Or her lower body could be pants. Who knows? We only know that she is topless.

Besides, if I searched for nude, I'd like to see a character fully in the nude, not just their upper half. That may be just me, but that's how I would use that search term.

I agree with everyone. Topless because although the picture has no clothes on, you can't see her bottom...she could theoretically be wearing pants or something.