In some TV shows, in order to develop a character, there may be some episodes that are not closely related to the main story, e.g., an episode of the background story of a supporting character explaining his motivation or why he becomes the very person he is.
What do you call an episode of this type?
Another term that is commonly used for this situation would be a "filler episode." However, in this case, the term means that the episode is not considered important viewing for the overall plot. It's possible that a side character's motivation may not be considered filler if the viewer considers it to be important.
"Filler episode" also has a more negative connotation than "standalone episode." The former implies that the viewer could skip the episode if they choose, while the latter does not.
If it doesn't fit neatly into an overall story arc, then it is often called a "standalone episode" (or a "self-contained episode"). It may also be described as "episodic".
Another term for this is non-arc episode, one that is not part of the story arc.
You could call them monster of the week episode.
An episode that develops the overall setting of the story, but not the story itself, may be called "worldbuilding". An episode that doesn't have any usefulness at all, other than taking up time, is referred to as "filler". Opinions may vary as to which category particular episodes fall in.
Offshoot
"a side shoot or branch on a plant" or "a thing that originated or developed from something else"
Informally I've heard this called a "character arc", though everything I can find online says that formally, that term should refer to a character's development rather than specific episodes or a specific section of the book.
