What's the definition of a pathological liar?

Clinically, there’s probably no real difference. “Pathological lying” is not a diagnosis, it’s one sign of a different mental health diagnosis. ‘Pathological” may indicate that the lying is more destructive to the person and his/her relationships, but really, pathological just sounds more medical, and thus worse.

I’ve always understood (simply put) is that the difference is that a pathological liar is not just someone who lies compulsively, but more importantly actually starts to believe the lies they’re telling. More about it is a simple wiki search away:

Mythomania: Wikipedia

Thanks Spargett, this is a great distinction.