A decal is an image that can be transferred to a surface using heat or water. Decals are similar to stickers, but they don't use a sticky substance for adhering to the new surface.
A decal is sometimes called a "transfer," and it consists of a picture, image, pattern, or design that's printed on paper, plastic, or cloth. By holding a hot iron on the back of the decal, or dabbing a small amount of water on it, the image can be moved to a new surface. Decal is a shortened form of decalcomania, from the original French décalcomanie, a technique for transferring prints onto pottery.