According to W.H.O. depression is 4th most common disease on the world after arthritis, heart diseases & respiratory diseases.

In revised statement on June 2020. W.H.O. says depression is now most common disease in the world.

As per American Psychological Association form criteria in DSM 5(DIGNOSTIC STATICAL MANUAL) define symptoms of depression are:

Sad feelings or low mood.
Decreased interest or pleasure in almost everyday activities.
Weight loss or decreased appetite.
Weight gain or increased appetite.
Sleep disturbances.
Slowing in all activities, fatigue and irritability.
Feeling of worthlessness.
Recent thoughts about suicide/death.