Graffiti
Unsanctioned writing or images applied to surfaces in public view, a category that runs from simple tagging to elaborate lettering styles carrying a half-century of subculture.
The graffiti-versus-street-art distinction is contested, but the working version goes: graffiti traditions centre lettering and the writer's name, addressed in large part to other writers; street art typically centres imagery addressed to the general public. Permission is a separate axis: plenty of celebrated work began unsanctioned, and plenty of sanctioned work borrows graffiti's style.
Municipal responses span rapid-removal policies, restorative-justice programs, legal walls, and commissioning pipelines that hire from the culture rather than against it. Most cities run several of these at once, which is less contradictory than it sounds: tagging a heritage façade and painting a masterpiece on a sanctioned wall are different acts, and policy can say so.