Unfortunately, there is really no agreed-upon definition of what a mental disorder is.
Here is the outline that the developers of the DSM-IV-TR (2000) used as their guide (as we learn more about mental disorders, these criteria are subject to change):
A clinically significant behavioral or psychological syndrome or pattern that occurs in an individual and that is associated with present distress, disability, or a significantly increased risk for suffering.
A mental disorder is basically a way of thinking, behaving, or acting that causes distress in a person’s life. This does not include grief over the loss of a loved one or hardships beyond an individual’s control (such as a flood), unless the reaction of the person goes beyond what would normally be expected for someone in the same situation.
Although the overall definition of a mental disorder may be difficult to pin down, there are definitive ways to classify individual disorders, as discussed in subsequent questions and answers.