The term is most widely used in anime fandoms, where filler more precisely refers to anything that isn't in the original source material, as the vast majority of anime aren't wholly original works but rather adaptations of existing material.
This could be considered Padding (the addition of scenes to lengthen a story) applied to a whole franchise by creating brand new episodes. Filler episodes are entries in a generally continuous serial that are unrelated to the main plot, don't significantly alter the relations between the characters, and generally serve only to take up space.
