Depression ranges in seriousness from mild, temporary episodes of sadness to severe, persistent depression. Clinical depression is the more-severe form of depression, also known as major depression or major depressive disorder. It isn't the same as depression caused by a loss, such as the death of a loved one, or a medical condition, such as a thyroid disorder.

To diagnose clinical depression, many doctors use the symptom criteria for major depressive disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published by the American Psychiatric Association.

Signs and symptoms of clinical depression may include:

• Feelings of sadness, tearfulness, emptiness or hopelessness
• Angry outbursts, irritability or frustration, even over small matters
• Loss of interest or pleasure in most or all normal activities, such as sex, hobbies or sports
• Sleep disturbances, including insomnia or sleeping too much
• Tiredness and lack of energy, so even small tasks take extra effort
• Reduced appetite and weight loss or increased cravings for food and weight gain
• Anxiety, agitation or restlessness
• Slowed thinking, speaking or body movements
• Feelings of worthlessness or guilt, fixating on past failures or self-blame
• Trouble thinking, concentrating, making decisions and remembering things
• Frequent or recurrent thoughts of death, suicidal thoughts, suicide attempts or suicide
• Unexplained physical problems, such as back pain or headaches

Symptoms are usually severe enough to cause noticeable problems in relationships with others or in day-to-day activities, such as work, school or social activities.

Clinical depression can affect people of any age, including children. However, clinical depression symptoms, even if severe, usually improve with psychological counseling, antidepressant medications or a combination of the two.