A broad genre of visual communication placed on surfaces, usually in urban environments; term reserved for works that are considered art or artistic, rather than casual notations or scribbles, which are called "graffiti." Examples of graffiti art include works expressed in a signature form, in either a stylized monogram or as large colorful murals. Examples may include works that are not in graphic media or are not created directly on the surface, but created and then applied to the surface. Graffiti art has occasionally been created as paintings on moveable supports. Contemporary graffiti art emerged in the mid-1970s in New York City, rendered in spray paint on subway train cars and walls. Originally graffiti art was illegal, applied to unauthorized public space or property by individuals or groups, though now it may also appear as commissioned work.

Source: AAT

http://vocab.getty.edu/page/aat/300410270