Someone who identifies as one or more non-human animal(s) on a personal, non-physical, integral level. Someone can be a therianthrope for spiritual reasons, psychological reasons or a mixture of both spiritual & psychological reasons. A connection on its own doesn't make someone a therianthrope. That would just be animal-hearted. Therianthropes do not have to feel a connection to or even like the animal(s) they identify as.
Therianthropes are not delusional or mentally ill. For therianthropy to be a delusion, it has to contradict external reality. Therianthropes will acknowledge that they are human, this is why therianthropes are not delusional. For therianthropes to be mentally ill, therianthropy actually has to hinder functioning. Again, most therianthropes function just fine.
The author of this definition is a coywolf therianthrope.
If you want to learn more about therianthropes & therianthropy, tumblr is not a good source.