Major Depressive Disorder is a mental health disorder clinically diagnosed by certain health practitioners including psychiatrists and some MDs. Diagnosis comes mostly from assessing whether the patient has had many of the symptoms associated with depression for more than 2 weeks. The Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) lists 9 symptoms commonly associated with depression, and if a person exhibits 5 of these for at least a 2-week period, they are said to have major depression (or clinical depression). FoundHealth has an article on this: Depression Diagnosis.
Minor depression is technically categorized as having only 2 of the 9 symptoms outlined in the DSM-IV.