A psychological disorder is basically any pattern of thoughts or behavior which leads to subjective distress, which significantly interferes with the person's life, and which has characteristics that are recognizable as a diagnostic category by a mental health professional. In other words, if you have a psychological disorder, there are other people who also have or have had it. It also has to be seen as "abnormal."
For example, if you are very sad about your dog dying, you may seem no different from a person who is suffering from the psychological disorder of "depression." However, because it seems that being sad when your dog dies is "natural" you would not be diagnosed as having a "depressive disorder."
In addition, some "abnormal" behaviors can be reinterpreted to seem normal. For example, Larry Bird, the famous basketball star for the Celtics, shot 2000 free throws every morning before breakfast. He was seen as "dedicated" and "highly motivated". If you or I did that, they would say we were suffering from an obsessive-compulsive disorder. Myself, I think anyone who would get up before dawn to shoot 2 thousand free throws is nuttier than a fruitcake, but that's me.
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