OVA

Original video animation (オリジナル・ビデオ・アニメーション, Orijinaru bideo animēshon^?), abbreviated as OVA (オーブイエー / オーヴィーエー / オヴァ, ōbuiē, ōvīē or ova^?) media (and sometimes as OAV, original animated video), are animated films and series made specially for release in home video formats without prior showings on television or in theatres, though the first part of an OVA series may be broadcast for promotional purposes. OVA titles were originally made available on VHS, later becoming more popular on LaserDisc and eventually DVD. Starting in 2008, the term OAD (original animation DVD) began to refer to DVD releases published bundled with their source-material manga.

Like anime made for television broadcast, OVAs sub-divide into episodes. OVA media (tapes, laserdiscs, or DVDs) usually contain just one episode each. Episode length varies from title to title: each episode may run from a few minutes to two hours or more. An episode length of 30 minutes occurs quite commonly, but no standard length exists. In some cases, the length of episodes in a specific OVA may vary greatly, for example in GaoGaiGar FINAL, the first 7 episodes last around 30 minutes, while the last episode lasts 50 minutes; the OVA Key the Metal Idol consists of 15 separate episodes, ranging in length from 20 minutes to nearly two hours each; The OVA Hellsing Ultimate had released 10 episodes, ranging from 42 minutes to 56 minutes. An OVA series can run anywhere from a single episode (essentially a direct-to-video movie) to dozens of episodes in length. Probably the longest OVA series ever made, Legend of the Galactic Heroes, spanned 110 main episodes and 52 gaiden episodes.

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Wikipedia