From The Art and Popular Culture Encyclopedia
An episode is a part of a dramatic work such as a serial television or radio program. An episode is a part of a sequence of a body of work, akin to a chapter of a book. The term sometimes applies to works based on other forms of mass media as well, as in Star Wars. Episodes of news programs are also known as editions.
The idea of stories being told in episodes has origins in serialized literature, and in Aristotle's Poetics as "pataka". Another early example of this is the One Thousand and One Nights (Arabian Nights), which consisted of a series of serialized stories, or "serialized novels" or novellas.
Shows usually have numbers or codes (aka, Production codes/numbers) for each episode. The X-Files, for example, assigned a code in the format "sXnn", with "s" identifying the season number and "nn" being a two-digit number for each episode, starting with "01".
In addition, the word episode can also refer to a portion of a tragic play; this usually being associated with those of the ancient Greeks.