Inscriptions, figure drawings, etc., found on the walls of ancient sepulchers or ruins, as in the Catacombs, or at Pompeii.  [1913 Webster]

 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]