The official definition of depression (from the DSM-IV TR—the American Psychiatric Association's diagnostic manual) is the presence of at least one of the following three abnormal moods which significantly interfere with the person's life:

• Abnormal depressed mood most of the day, nearly every day, for at least two weeks;
• Abnormal loss of all interest and pleasure most of the day, nearly every day, for at least two weeks;
• If 18 years or younger, abnormal irritable mood most of the day, nearly every day, for at least two weeks.

In addition, at least five of the following symptoms must have been present during the same two week depressed period:

• Abnormal depressed mood (or irritable mood if a child or adolescent) as defined above;
• Abnormal loss of all interest and pleasure as defined above;
• Appetite or weight disturbance, either, abnormal weight loss (when not dieting) or decrease in appetite or abnormal weight gain or increase in appetite;
• Sleep disturbance, either abnormal insomnia or abnormal hypersomnia (excessive sleeping);
• Activity disturbance, either abnormal agitation or abnormal slowing (observable by others);
• Abnormal fatigue or loss of energy;
• Abnormal self-reproach or inappropriate guilt;
• Abnormal poor concentration or indecisiveness;
• Abnormal morbid thought of death (not just fear of dying) or suicide.