What is the definition of pathology? While trying to get more information for my previous question, I found several different definitions for the terms "pathology" and "pathological." While under normal circumstances I'd be inclined to say the latter stems from the former, there seems to be heavy connotation where terms such as "pathological liar" are concerned.

One of the definitions at dictionary.com states:


  Caused by or evidencing a mentally disturbed condition


Seems a bit vague, to me. Can anyone clarify this?
I think the answer can actually be found in the Dictionary.com link you cite:


  -ological; suffix;
  used to form adjectives; belonging or relating to a particular type of scientific study;
  e.g. biological, technological


And (taken from Wikipedia):


  The word pathology is from Ancient Greek πάθος, pathos, "feeling, suffering"; and -λογία, -logia, "the study of".


So pathological would be:


  an adjective describing a thing that belongs or relates to suffering or the feeling of suffering.  


(literally translated, of course)

So we can see that it's not just disease-related (suffering) but also mental health-related (feeling of suffering).