The term filler refers to anything outside the continuity of the series in question, even though it may seem "official". In the OBD feats from fillers are usually entirely disregarded as non-canon. It is rarely used save for instances where the OP allows it. Filler is most often used to refer to episodes of an anime which were not in the manga, if the manga is the original canon. Dragon Ball and Naruto are examples of universes where filler was often mistaken for canon in debates in the early days of the OBD. Filler can also refer to alternate continuities (for example, the Tenchi Universe and Tenchi in Tokyo animes, which are not canon to Tenchi Muyo!)
Sometimes the term is also used to refer to canon material that seems to be pointless and doesn't contribute to the plot.