1 (usu. in pl.) a piece of writing or drawing scribbled, scratched, or sprayed on a surface.
Usage:
Not a mass noun in this sense, and so a plural construction is needed, e.g. graffiti are (not is) an art form.
2 Art a form of decoration made by scratches on wet plaster, showing a different-coloured under-surface.
Etymology: It. f. graffio a scratch
Useful english dictionary. 2012.