Major depression (also known as Major Depressive Disorder, Clinical Depression, and Unipolar Disorder) is a chronic mood disorder characterized by extreme sadness, lethargy, sleep disturbances, pessimistic thinking, and loss of interest in life. Although depression can occur normally after a traumatic event (such as the death of a loved one) or a medical condition (like thyroid problems) major depression is different in that it occurs in a long lasting fashion and isn't necessarily caused by life events.
To be diagnosed with major depression you must present 5 or more symptoms for longer than 2 weeks. The symptoms must be present almost every day and occur for most of the day. Major depressive symptoms are: depressed mood, reduced interest and pleasure, weight loss or gain because of altered appetite, sleep disturbances, altered behaviors (like restlessness), fatigue, feelings of guilt or weakness, decision making problems, and thoughts of death or suicide.
Treatment for major depression varies between individuals and typically includes therapy, medication, or a combination of the two. Also, when people use the general term, Depression, they are actually referring to Major Depression.