Depression

1. In psychology and psychiatry, the concept of depression refers to an affective disorder characterized by persistent low mood, loss of interest, decreased energy, and a range of cognitive, somatic, and behavioral symptoms. Diagnostic classifications specifically define and categorize this disorder, taking into account its variants, severity, and duration. In the ICD-11, depression is classified under codes 6A70 – Episodic depressive disorders and 6A71 – Recurrent depressive disorders.
2. A negative affective state, ranging from unhappiness and discontent to an extreme feeling of sadness, pessimism, and despondency, that interferes with daily life. Various physical, cognitive, and social changes also tend to co-occur, including altered eating or sleeping habits, lack of energy or motivation, difficulty concentrating or making decisions, and withdrawal from social activities. It is symptomatic of a number of mental health disorders (2).

Source:

(1) ICD 11 – WHO (2017)
(2) https://dictionary.apa.org/depression