What is a filler episode?

Answer:

A filler episode is generally an episode in which there is no plot development. Good filler episodes generally keep with the tone of the overall series while developing the original cast. Some examples of filler episodes may include, flashbacks and recaps where events up to the current point are once again stated or summarized, introduction of new and strange characters or sidestories though sometimes this may have influence in the outcome of the series as a whole, random conflicts that are resolved in a single episode are sure signs of a filler episode, and others. An exception to filler episodes include episodic animes where although an episode may seem out of place, since the genre of the series implies different ordeals per episode, the only real filler type in these shows would be flashbacks. A sure sign to tell that a filler has past is when after the episode or in some cases arc (examples of filler arcs include all of Naruto season 1 after visiting Orochimaru's castle and the Bounto/Bound arc in Bleach) the main characters show no real imporvement. In the examples given, Naruto and friends do not gain any real new skills nor do the battles really change their views. In Bleach, the Bounto/Bound arc merely mirrored the events of the previous Soul Society arc and subsequently only strengthened the resolve of the main characters but in hindsight was not really necessary.

Another common definition of a filler generally is that it is any plot event that does not coincide with its manga source. However, since the TV series is usually an adaptation, this is not always the case, but in the case of Shounen Jump series animes/manga there is a close enough tie between the two mediums that the definition is widely accepted.