Definition of Depression

Depression: An illness that involves the body, mood, and thoughts and that affects the way a person eats, sleeps, feels about himself or herself, and thinks about things. 

Depression is not the same as a passing blue mood. It is not a sign of personal weakness or a condition that can be wished away. People with depression cannot merely 'pull themselves together' and get better.

Without treatment, symptoms can last for weeks, months, or years. Appropriate treatment, however, can help most people with depression. 

The signs and symptoms of depression include loss of interest in activities that were once interesting or enjoyable, including sex; loss of appetite, with weight loss, or overeating, with weight gain; loss of emotional expression (flat affect); a persistently sad, anxious, or empty mood; feelings of hopelessness, pessimism, guilt, worthlessness, or helplessness; social withdrawal; unusual fatigue, low energy level, a feeling of being slowed down; sleep disturbance and insomnia, early-morning awakening or oversleeping; trouble concentrating, remembering, or making decisions; unusual restlessness or irritability; persistent physical problems such as headaches, digestive disorders, or chronic pain that do not respond to treatment, and thoughts of death or suicide or suicide attempts. The principal types of depression are called major depression, dysthymia, and bipolar disease (manic-depressive disease).

http://www.medicinenet.com/depression/article.htm