A durable and flexible material created via the tanning of hides.
In appearance, it is often vaguely similar to latex, but with the softer shine and higher value range that come with more spread out light reflection, a distinguished pattern, and less skin-tight. The pattern is not a requirement however, and won't be visible in most images on danbooru anyway. Likewise, it will often be drawn more skin-tight than expected.
There are various forms of leather, including ones with plastic finishes, making distinguishing between leather and latex rather hard at times, especially in anime-style art.