episode

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An incident or event that is part of a progression or a larger sequence.
• noun One of a series of events in the course of a narrative or drama. synonym: occurrence.
• noun A separate part of a serialized work, such as a novel or television series.
• noun A section of a classic Greek tragedy that occurs between two choric songs.
• noun Music A passage between statements of a main subject or theme, as in a rondo or fugue.

from The Century Dictionary.

• noun A separate incident, story, or action introduced in a poem, narrative, or other writing for the purpose of giving greater variety; an incidental narrative or digression separable from the main subject, but naturally arising from it.
• noun An incident or action standing out by itself, but more or less connected with a complete series of events: as, an episode of the war; an episode in one's life.
• noun In music, an intermediate or digressive section of a composition, especially in a contrapuntal work, like a fugue.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Rhet.) A separate incident, story, or action, introduced for the purpose of giving a greater variety to the events related; an incidental narrative, or digression, separable from the main subject, but naturally arising from it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An incident or action standing out by itself, but more or less connected with a complete series of events.
• noun An installment of a drama told in parts, as in a TV series.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a happening that is distinctive in a series of related events
• noun a part of a broadcast serial
• noun film consisting of a succession of related shots that develop a given subject in a movie
• noun a brief section of a literary or dramatic work that forms part of a connected series