t-shirt material
Any image that is contained inside its borders without the image ending at said borders in such a way that the space between the image and its border is one, uniform colour (normally white or black, since those are the most common plain t-shirt colours), making for an ideal non-square image that can be pressed onto a custom shirt. Watermarks or other writings in the corner of such image may be overlooked for this tag, as long as they don't join the main image.
Thus, post #717052 narrowly averts this tag.
Not the same as simple_background in that the image must make for a good t-shirt design and the image inside the uniform background won't necessarily have a simple background itself.
Thus, post #716851 will overlaps simple_background and t-shirt_material, while post #587741 doesn't.
T-shirt designs should, under common sense, avoid being explicit, though visual puns and whatnot may override this ruling.