Definitions for "OVA/OAV"
Original Video Animation, aka straight‑to‑video. Sometimes these productions are standalone series, other times they are used to test the market to see if the story is popular enough to warrant a TV series. They are also sometimes used to round out a TV series by providing “bonus” episodes containing content which, due to time constraints or its graphic nature, could not be aired on TV. OVA series rarely run more than 5-6 episodes and have reportedly become less common in recent years in Japan.
Stands for Original Video Animation/Original Animated Video, i.e. an anime that is released on video and not TV or Movie.
Original Video Animation/Original Animation Video respectively. Both terms are interchangeable. This is animation which isn't aired on TV and instead is released on VHS, Laser Disc or DVD only. Production values are higher than TV series' but tend to be lower than theatrical releases. Normally they only span a few episodes as opposed to a TV series which spans many episodes.