Depression Definition and DSM-5 Diagnostic Criteria, [1]
The DSM-5 outlines the following criterion to make a diagnosis of depression. 

A. The individual must be experiencing

  1. five or more symptoms during the same 2-week period and 
  2. at least one of the symptoms should be either: 

    (a)  depressed mood or 
    (b)  loss of interest or pleasure

B. Symptoms:
1.  Depressed mood most of the day, nearly every day.
2.  Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day.
3.  Significant weight loss when not dieting or weight gain, or decrease or increase in appetite nearly every day.
4.  A slowing down of thought and a reduction of physical movement (observable by others, not merely subjective feelings of restlessness or being slowed down).
5.  Fatigue or loss of energy nearly every day.
6.  Feelings of worthlessness or excessive or inappropriate guilt nearly every day.
7.  Diminished ability to think or concentrate, or indecisiveness, nearly every day.
8.  Recurrent thoughts of death, recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide.

C. To receive a diagnosis of depression, these symptoms must:

  1.  cause the individual clinically significant distress or impairment in social,
  2.  occupational, or 
  3.  other important areas of functioning. 

* The symptoms must also not be a result of substance abuse or another medical condition.