Decal
From Brickwiki
A decal 1 is a printed design on a medium (usually a special paper) that can be separated from a backing paper and moved to another surface upon contact with water 2.
Although similar to a sticker, decals allow for more detailed printing and are available on white or clear medium. This provides for much more versatility in design. As well, the medium is thinner and adheres more closely to the surface of the chosen element. Thus, especially in the case of the clear type, creating a look closer to genuine printed parts. Decals and stickers are very popular with customisers and given that LEGO itself uses stickers it is an area where purists may make an exception to their objections to non-LEGO additions.
LEGO makes no use of decals as they either print directly or provide a sheet of pre-printed and perforated stickers which the builder must apply.
1 - also called a transfer, water-slide decal or water-slide transfer.
2 - for more technical discussions of decals see the Wikipedia article.
