Definition of “depression” - Learner’s Dictionary

depression

noun [ C, U ] uk us /dɪˈpreʃən/
UNHAPPINESS

B2 the feeling of being very unhappy for a period of time, or a mental illness that makes you feel very unhappy:

Nearly three million people suffer from depression every year.
ECONOMY

a time when there is not much business activity:

The stock market crash marked the start of a severe depression.

(Definition of “depression” from the Cambridge Learner's Dictionary © Cambridge University Press)