The Collaborative International Dictionary of English v.0.48:

Graffiti \Graf*fi"ti\, n. pl. [It., pl. of graffito scratched]
   1. Inscriptions, figure drawings, etc., found on the walls of
      ancient sepulchers or ruins, as in the Catacombs, or at
      Pompeii.
      [1913 Webster]

   2. any writings or drawings on a surface in a public place,
      placed there without authorization of the owner of the
      object on which it is written. Such graffiti are usually
      unwelcome, and are considered as form of vandalism.
      [PJC]

WordNet (r) 3.0 (2006):

graffiti
    n 1: a rude decoration inscribed on rocks or walls [syn:
         graffito, graffiti]

The Free On-line Dictionary of Computing (30 December 2018):

Graffiti

   Handwriting recognition software for the Newton and Zoomer
   which recognises symbols that aren't necessarily letters.
   This gives greater speed and accuracy.  It was written by
   Berkeley Softworks.

   (1995-01-24)