A "turning" is a term used within the mythical community to refer to the event in which an unturned mythological creature takes their true form by breaking any preexisitng illusion preserving their human form. These illusions are either inherited from a medallion-using, human disguised ancestor, or from a monster given a human form through the use of non-medallion magic.
The use of medallions are the cause of the most common type of turning, in which a disguised creature is either given or somehow acquires a medallion that corresponds to their specific species. Once this occurs, the medallion will replace the old spell disguising them as a human with a new one. This gives the newly-turned mythological creature the ability to shift to any shape between their natural and human forms, although it usually takes some practice before one can achieve a stable midform.
Natural turnings are when an unturned creature somehow manages to break their illusion of humanity and revert to their natural form without the aid of a medallion. Natural turning is possible, but uncommon. Examples of creatures turning in this manner include Merial McMinnus, Isaac "Ike" Sanford, and Anthony Gillis, with the latter two being monsters and inheriting their human appearances through unconventional means. Natural turnings, unlike medallion assisted turnings, will displace clothing rather than magically storing any articles of clothing that might become impractical, unecessary, or improperly fitting. This resulted in shredded pants in the case of Merial, who's turning resulted in a single tail instead of two legs, as well as Anthony's shirt tearing along the sides due to his arms expanding significantly in their transition to wings.
Natural turnings in the case of monster given a human appearance also tend to take significantly longer and are much more uncomfortable than the instant, painless turnings of creatures who inherited the illusion spell from their parents. These turnings are actually forms of physical shapeshifting, and unlike the medallion-driven turnings, can take anywhere from mere minutes as in the case of Ike, to an entire day as in the case of Anthony.
It was also mentioned on Sheana Molly's Deviantart page
that Iris Ophis, a lamia, had a natural turning partway through her teenage years, although this has not been mentioned in the comic as of the end of Illumination.