From The Heretic Knowledge Vault
Term for "bonus" material going beyond the bounds that a product normally promises. The term is derived from the Japanese; for a full explanation, check this Wikipedia article.
Filler art generally qualifies as omake, and the recurring Fun with Familiars and Does Not Play Well With Others series are definitely omake, at least unless/until they spin off as full-blown serials of their own. (Note that this has happened with DMPWWO, which see.) In addition, images of Miyo Hinadori (the one used to illustrate her article) and Riley (lost for the moment -- warning, definitely not work-safe) have appeared as omake previously. There'll be more.
Note that the specific category of omake known as the Chronicles of Heretic Knowledge are now a regular Saturday feature of the strip, and will remain such as long as Errant Story itself is updating on a twice-weekly, rather than thrice-weekly, schedule.