I disagree with tagging bikini or bikini_top because it is not sufficiently clear from this picture alone that the character is in fact wearing one or both. It could absolutely be a one-piece swimsuit or even a sundress; without the rest of the picture, whatever garment she is wearing could go all the way down to her thighs or even her ankles.

Even if this is one of a series of pictures where the girl IS wearing a bikini, we at Danbooru are supposed to tag what we see.