A decal (pronounced /ˈdiːkæl, ˈdiːkəl, ˈdɛːkl/) or transfer (chiefly British usage) is a plastic, cloth paper or ceramic substrate that has printed on it a pattern that can be moved to another surface upon contact, usually with the aid of heat or water. The word is short for "decalcomania." The word Decalcomania is derived from the French word "decalquer," and was coined by Simon François Ravenet about 1750. The "mania" was added during the Decal craze of the late 1800s.

Different variations of decals include: "water-slide" or "water-dip"; and vinyl "peel-and-stick". A water-slide (or water-dip) decal is screen-printed on a layer of water-soluble adhesive on a water-resistant paper, that must first be dipped in water prior to its application.. Upon contact with water, the glue is loosened and the decal can be removed from its backing; overly long exposure, however, dissolves the glue completely causing the decal to fail to adhere. A "peel-and-stick" decal is actually not a decal as described above, but a vinyl sticker with adhesive backing, that can be transferred by peeling off its base.

Decals are commonly used on hot rod automobiles and plastic models. They are also used on guitars as a way of personalizing them.

Government agencies of all types also use decals on vehicles for identification. These decals are referred to as fleet markings and are required by law on all fire and law enforcement vehicles in the US. Most fleet markings are created from reflective vinyl with an adhesive backing that is applied in a peel-and-stick manner. Vinyl comes in large rolls that are fed through a plotter (cutter) or large-format printer/cutter. The designs are created in specialized computer software and sent to the machines via cable link for production. Once the design is cut into the vinyl, the excess vinyl on the sheet is removed in a process called "weeding". Finally, a paper pre-mask is applied to the top of the vinyl design to allow easy application of multiple letters and shapes at one time.