Images or animations depicting a character who is wearing clothing over the lower half of their body, but not the upper half. The equivalent of bottomless.
Toplessness is only considered questionable in nature if the shirtless character has breasts. However, for the sake of thoroughness, the tag should also be applied to other characters as well. It's added regardless of gender, based just on being partially clothed.
This should be obvious, but there have been mistakes in the past, so to be clear: do not apply both the "bottomless" and "topless" tags to an image in which the character is nude.