MOTW stands for "Monster of the Week." It was originally used to refer to X-Files episodes that were more standalone, and separate from the series' overarching mythology or "mytharc."
It has spread throughout fandom, used in discussion of everything from Buffy to Supernatural fandoms.
Examples of Use
She began writing fan-fiction
in the middle of the Third Season when the show began to drift away from these themes and arcs. Her stories, generally case files which are a combination of myth-arc
and MOTW, all grow in one sequence. 
- ^ July 1998 at ARCHIVISTS AND AUTHORS! - Mary Ruth Keller