OVA

OVA is an abbreviation for Original video animation (オリジナル・ビデオ・アニメーション) which refers to Japanese animated films and series (anime) which are made specially to be released on home video formats. For more information, please see Original video animation.

In terms of Sonic, 'OVA' usually refers to Sonic the Hedgehog: The Movie, a two part OVA.