When people use the term clinical depression they are generally referring to major depressive disorder (MDD).

Major depressive disorder is a mood disorder characterized by a number of key features:

• Depressed mood
• Lack of interes...

From a medical standpoint, depression is defined​ as a mood disorder that causes a persistent feeling of depressed mood or sadness and the often profound loss of interest in things that usually bring you pleasure.

Depression affects how you feel, think, and behave and can i...

Premenstrual dysphoric disorder (PMDD) produces similar symptoms, but those related to mood are more pronounced.

PMDD symptoms may include:

• Extreme fatigue
• Feeling sad, hopeless, or self-critical
• Severe feelings of stress or anxiety
• Mood s...

Seasonal affective disorder (SAD) is a mood disorder that is characterized by symptoms that occur at the same time each year, usually during the darker, shorter days of fall and winter.

Currently called major depressive disorder with seasonal pattern.

SAD is believed ...

People might experience brief periods of not feeling depressed, but this relief of symptoms lasts for two months or less. 

While the symptoms are not as severe as major depressive disorder, they are pervasive and long-lasting.

PDD symptoms include:

• Feelings...

Bipolar disorder is a mood disorder characterized by periods of abnormally elevated mood known as mania. 

These periods can be mild (hypomania) or they can be extreme. The vast majority of those with bipolar disorder also have episodes of ...

So all together you have:

1. Major depressive disorder (MMD)
2. Seasonal affective disorder (SAD)
3. Atypical depression
4. Persistent depressive disorder (PDD)
5. Bipolar disorder (BPD)