Depression, clinically called major depressive disorder, is characterized by a significantly depressed mood, low energy, and low interest in normally enjoyable hobbies more than half of the time for a period of several months. The depressed mode characteristic of the disorder does not necessarily refer to sadness, but could also mean hopelessness, helplessness, worthlessness, restlessness, or a lack of any emotion at all. This varies from individual to individual.

Depression is one of the most misunderstood mental disorders. Many people do not understand that there are physical components to depression as well as mental ones, which leads to misunderstanding and may prevent people suffering from depression from seeking help. People who have a loved one with depression are encouraged to think of it as a chronic physical condition like diabetes that requires ongoing maintenance as well as support from friends and family. People with depression often feel worthless or guilty due to their condition, so it is important to be understanding of those with depression and support them in their treatment.
Depression has several symptoms that are not commonly known, most notably the inability to experience pleasure from activities that were previously enjoyed. Depression causes rumination over feelings of guilt, regret, or worthlessness. Poor concentration, withdrawal from social activities, reduced sex drive, insomnia, or hypersomnia are common symptoms of depression. Children with depression exhibit irritability rather than a low mood.
There is no conclusive evidence about the causes of depression, but it is thought to be caused by a combination of biological, social, and environmental traits. It was once thought that depression was caused by an imbalance of serotonin in the brain, but that has since proven to be not fully true. People with depression show different brain composition than those of healthy people, though it is unknown whether this is a cause of depression of a result of it. Social isolation and poverty are correlated with depression as well as certain personality types. People with depression often have low self-esteem, distorted thinking, and negative emotionality. Negative life events can also trigger a depressed mood state, but when the depressed mood lasts more than a few months, it can turn into major depressive disorder. Drug and alcohol is another contributing factor to depression.
The first line of treatment for depression is therapy; cognitive behavioral therapy (CBT), in particular, has shown the most promise. CBT seeks to identify and remove perpetuating, self-defeating thoughts and change behaviors. Anti-depressant medication should be prescribed if therapy alone does not fully manage the depression. Selective serotonin re-uptake inhibitors (SSRIs) are the most common family of anti-depressants and includes Zoloft (sertraline) and Prozac (fluoxetine).
As a last line of treatment, electroconvulsive therapy (formerly called electroshock therapy) is about 50% effective for treating treatment-resistant mood disorders. The procedure electrically induces seizures and is repeated two to three times per week until symptoms are relieved. Electroconvulsive therapy (ECT) is performed while the patient is under anesthetic and given a muscle relaxant.
Overall, about half of people who have a major depressive episode recover, while half will have another episode later in life. About 3.4% of individuals with depression die from suicide.