Depression is a condition that is characterized by a sense of 'feeling low' with reduced concentration, reduced energy and reduced ability to enjoy daily activities for at least two weeks. In simple terms, if a person feels low off and on and the symptoms can change within hours or one or two days, then the diagnosis of depression as an illness is inappropriate.
If depression becomes worse there can be a loss of appetite (in some cases an increase) with consequent loss in weight (or gain), bowel and bladder disturbances including constipation and loss of sexual drive or libido. If the condition become severe, a person may become suicidal with a sense of hopelessness.
In psychiatry, there is an acceptance of an underlying family or genetic element in the causation of this condition. It means that if a parent or a grandparent have or had depression then the condition can occur in younger generations. In psychology, it is believed that an individual suffering with depression learns habits or gets 'conditioned' to think and behave like a depressed person. Treatment in the two approaches differs. Psychiatry focusses on the 'chemical imbalance' hypothesis of depression and so relies on medications for treatment. Psychological intervention involves changing the habits of thinking and behaving in a particular - depressed- way. Current western prevalent mode of psychotherapy focusses more on CBT or cognitive behaviour therapy, though other kinds of psychotherapy modalities are available that could be equally effective.
My own experiences of treatment of such condition is with and without medication. In many cases, I have had to help a person come off medication gradually, which can be done under supervision along with psychotherapy. The way I work with my clients is with imagery and meditation or simple breathing exercises.
In eastern parts of the world, the symptoms of depression are more physical than psychological. A person may not be able to express themselves verbally because of family pressures and demands of the culture. So the symptoms appear as pains and aches in various parts of the body. They may also appear as anxieties and fears. Though grief is different from depression, in my experience, too many losses of family members, friends or relations can cause the condition of depression even many years later after the events.
In my view, meditation can be successfully used (when done regularly) to treat depression under the supervision of a professional who has experience in dealing with such cases.