Pictures where the character(s) seem to stand on scenic bases, similar to that of a figurine or scale model. In other words, the only real background is just a small chunk of ground or scenery.
If the background is cropped, but instead of having clear cuts with volume, they appear foggy or as if simply being erased, it's fading border instead.
A backgroundless character art removed from a faux figurine will likely become a tachi-e or invisible chair.
Images with a Vignette(ヴィネット) tag on pixiv should be tagged as "faux figurine" or "isometric" on aibooru.