A short video to promote a song, the visuals usually a mix of the artist performing and other imagery such as a story that goes with the song. Before the days of MTV the rock video was fairly unheard of, and was probably just called a 'promo clip'. The medium really flourished in the 80s, when MTV changed the payola structure of the music industry completely, but since then it has become more of an ordinary television network, featuring its crappy programming more than rock videos.