In the field of psychiatry, depression is a metal disorder. It is important not to confuse it with depressive feeling, such as a sadness and worry, which is part of the spectrum of emotions that every person experiences throughout the day.
Depression is dominated by a feeling despair, which is usually accompanied by hypersensitivity as well as other symptoms. These may be related to the sphere of thought, such as thoughts of worthlessness, self-loathing, death wishes, even thoughts of suicide. 
Difficulty in paying attention and concentrating is another frequent symptom experienced by a person with depression, a fact that intensifies the feeling of worthlessness. 
On a somatic level the person may experience decreased energy, fatigue and a multitude of other disturbances such as headache, muscular aches or pains, etc. 
A person with depression usually avoids coming into contact with other people experiencing unbelievable isolation as a result; he feels empty and nothing offers him happiness or pleasure.

Loss of appetite, resulting in weight loss, and sleeping disorders are some other symptoms that are regularly noted. 
Anxiety is a disorder that often accompanies depression.

Causes of depression:

Α) Biological factors.
- There may be a family history.
- Chemical imbalance.
- The incidence of endocrine disorders.

Β) Psychological factors .
- The fear of ending a relationship with a loved one.
- The feeling of forthcoming illness.
- Disregarding the need of developing and reinforcing self-esteem by the parental family.

C) Social factors.
- Unemployment.
- A sudden change in living conditions.

It is all too clear that depression is a biopsychosocial disorder. 

Treatment:

• Antidepressants.
• Psychotherapy methods.
• Electroconvulsive therapy under anaesthesia in medication-nonresponsive patients with depression has proven quite useful.

In the case of depression, it is appropriate to use the expression “every cloud has a silver lining”. If a person seeks medical attention from a professional he will not only recover, but through psychotherapy sessions he will develop skills that he did not have prior to ailing. 
He will acquire greater self-awareness and with the appropriate self-estimate, he will learn how to properly evaluate matters, relationships and situations that he is experiencing.