A transparent background; the ones that commonly appear in vector traces, pixel art and animated animated GIFs.
If the non-transparent part of the image is entirely in that background color, the result is a solid rectangle that gives no indication of the original image's content.
Make sure you're viewing the original image, as resized sample images are JPEG and will not have a transparent background.
Do not add simple background if the whole background is transparent.
The following tags are aliased to this tag: transparent_png, /transparentbg, transparent_gif, and /tbg (learn more).