Art created in public spaces, usually anonymous and unsanctioned. The term can include traditional graffiti, stencil graffiti, sticker art,  poster art, video projections, and street installations. Well known practitioners include Keith Haring (1958-1990), Jenny Holzer (b. 1950), Dan Witz (b. 1957), Shepard Fairey (b. 1970), Banksy and JR. Web resources here and here.

Keith Haring at work in the New York subway. 1980s.