Graffiti (noun)

1. Writing or drawings made on a public surface without permission.
2. Unauthorized writing or drawings scribbled, scratched, or painted on a public surface.

Origin:

From italian graffiti, plural of graffito, from graffiare ("to scratc.h")

Examples:

1. The graffiti on the wall said "No more war".
2. The train was covered in graffiti.
3. Graffiti is considered vandalism.
4. The city was plagued by graffiti.
5. Some people consider graffiti to be an art form.