Original Video Animation

Original video animation (オリジナル・ビデオ・アニメーション, Orijinaru bideo animēshon), abbreviated as OVA or sometimes as OAV (original animated video), are anime films or series made specifically only for home video format. In 2008, a new term OAD (original animation DVD) began to refer to DVD releases that are bundled with volume releases of their source-material manga.

History
OVAs originated during the early 1980s. VCR became a widespread fixture in Japan and the anime industry grew, creating massive demand for anime and "direct-to-video" became a necessity to keep up with the demand. Many popular series were like Tenchi Muyo! were released directly to video.

While originally, OVAs were released on VHS, as technology advanced, they later transitioned to other formats LaserDisc, DVD and Blu-ray.

After 2000, some shows included final episodes or side story episodes only in their home video release.

Benefits
Releasing works directly to video has its own benefits. As time goes, Japanese laws change, making some works needing censor or alterations. A lot of series are forced to censor material while airing on television, but are able to remove the censor for home release. Some shows had suffered backlash from their fans for the censors. Other shows were completely redone, one of the most popular examples is Hellsing and its OVA adaptation Hellsing Ultimate. The TV series met heavy backlash and disappointment from the fans. Hellsing Ultimate allowed the producers to make more faithful adaptation of the source-material and was met with positivity among the fans.

Producers started to use OVA for pilot episodes and test the interest in audience, and many popular series had started as OVAs and later become television series or movies. Some popular series continued only as OVA, like the original Hunter x Hunter adaptation. Nowadays some shows receive an OVA that is bundled with new volumes of the source-material manga, often between anime seasons to keep fans interest.

Trivia

 * The longest OVA series ever made, Legend of the Galactic Heroes, spanned 110 main episodes and 52 gaiden episodes.