Original animated video - Like a prologue; it's ususally made to give an origin to animes that have characteristics going unexplained in the series.
The Rurouni Kenshin OAV gives an explanation to the scar on Kenshin's cheek.
Original Animated Video
Anime made only for the video market. A small 'prolouge' to a series. Usually 4-6 episodes and short length. Usually the animation is different from the existing series.
Also called OVA
Did you the Samurai X OAV's?