According to the DSM-5, the latest edition of the Diagnostic and Statistical Manual of Mental Disorders, depression, or rather major depressive disorder, has two core characteristics:

• depressed mood(feeling sad and empty)
• markedly diminished interest or pleasure (not being able to experience joy or happiness)

For a diagnosis of major depressive disorder at least one of these two features needs to be present as well as at least four of the following seven signs and symptoms:

• feelings of worthlessness or excessive or inappropriate guilt
• inability to sleep or oversleeping
• significant weight loss when not dieting or weight gain
• fatigue or loss of energy
• diminished ability to think or concentrate, or indecisiveness
• psychomotor agitation or retardation(feeling restless or listless)
• recurrent thoughts of death or suicide and attempted suicide
   

If someone presents with at least five of these symptoms, including one of the two core symptoms, for a prolonged period (at least two weeks) and their daily functioning is evidently impaired, they can be said to be suffering from major depressive disorder.

Typically, people suffering from depression report major impairments in their everyday functioning, with their vitality, social and role functioning, and mental well-being being most severely affected. Their quality of life decreases equivalent to the severity of their symptoms. A depression often develops at a relatively young age and may become chronic, imposing a long-lasting burden. Depression is often associated with an unhealthy lifestyle (excessive smoking and alcohol intake, unhealthy diet), which increases the risk of various somatic conditions such as obesity, cardiovascular disease, and diabetes, while depressive disorder may also cause complex biological dysregulation that will in turn negatively affect physical health, reducing life expectancy and raising mortality. Together with its high prevalence, the disease burden of depression is inevitably substantial; it is ranked fourth according to the Global Burden of Disease project’sestimates.

Depression is more prevalent in people that have another mental disorder; anxiety disorders, substance use disorders (addiction) and personality disorders often co-occur with depression. People with a chronic somatic condition such as dementia, diabetes, Parkinson’s disease or stroke are also at higher risk of developing a depressive disorder, while depressive symptoms in older adults tend to be attributable to impaired functioning due to somatic illness.

Besides the classification of major depressive disorder, the DSM-5 also describes criteria for ‘persistent depressive disorder,’ defining chronic depression and a condition formerly known as dysthymic disorder.  People with persistent depressive disorder(previously dysthymia) have been suffering from a chronic depressive mood for at least two years, while in addition to this core symptom, they need to present with at least two of the following symptoms:

• poor appetite or overeating
• insomnia or hypersomnia (sleeplessness or excessive sleeping)
• low energy or fatigue
• low self-esteem
• poor concentration or difficulty making decisions
• feelings of hopelessness

Dysthymia was often erroneously considered to be a milder mood disorder than major depression, but its impact can be quite substantial by reason of its chronicity.