What is the definition of pathology?
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).