A webisode is an episode of a television show that airs initially as an Internet download or stream as opposed to first airing on broadcast or cable television. The word itself is a portmanteau formed by the words 'web' and 'episode'.
A webisode is simply a web episode – collectively it is part of a web series, a form of new media that characteristically features a dramatic, serial storyline, where the primary method of viewership is streaming online over the Internet. Where there is no "set" standard for length, most webisodes are relatively short, ranging from 4-15 minutes in length.
Source - Direct copy/paste from Wikipedia....