Depression
Depression (i.e. major depressive disorder) is a common and serious medical illness that adversely affects how you feel, think and act. It is basically a mood disorder that makes you always feel sad or feel a lack of interest in life and the activities you once enjoyed.

Depression can have many causes and risk factors, of which the more common ones include:

Depression not only leads to a variety of other emotional problems, but also issues with physical health that can decrease your ability to function at work and at home. Fortunately, it is also treatable. But for that to happen, the symptoms must first be recognized for you to be diagnosed.

People suffering from depression experience symptoms related to their:

These symptoms range from mild to severe, must last at least two weeks, and must represent a change in your previous level of functioning for a confirmed diagnosis. Since most cases of depression involve thoughts of suicide or self-harm, it is important to diagnose and treat depression on time.
What is depression? – Helen M. Farrell
How To Cope With Depression
Insight Into Depression – Sadhguru