Otherkin is a subjective and personal identification, in which someone identifies (non-physically) as another non-human entity on a deep, integral level. Not all non-human systemmates are otherkin; the label is a personal preference to those who feel it may fit them.
There are many subcommunities under the otherkin label. Therianthropy is a similar and well known identity to otherkin, and this is when someone identifies (non-physically) as a non-human animals on a deep, integral level.
What a therian or otherkin identifies as is called a theriotype or kintype, respectively.
Some otherkin may experience species dysphoria, a phenomenon that occurs when the physical body they reside in does not match their personal identity. This can be a distressing, uncomfortable feeling at times, and some may take steps to reduce species dysphoria through various means, one being shifting.
Shifting is a common experience to many otherkin and therians. A shift for therians and otherkin is any non-physical change in mentality or feelings that skew towards a non-human mode of being. Some otherkin may think more like their kintype at certain times, (called a mental shift) or feel body parts related to their kintype that are not physically there, termed phantom limbs. Others may feel their aura shift, senses subjectively change to mimic how their kintype might experience the world, or even shift into another creature that is not their kintype altogether.
Otherkin are a distinct movement from fictionkin, the (non-physical) identification as a fictional character on an integral and personal level. While they are similar, fictionkin are usually considered separate due to the lack of a non-human identity in the way otherkin have.