Depression, in everyday language, refers to any downturn in mood, which may be relatively transitory and perhaps due to something trivial. This is differentiated from clinical depression which is marked by symptoms that last two weeks or more and are so severe that they interfere with daily living.
In the field of psychiatry the word depression can also have this meaning but more specifically refers to a mental illness when it has reached a severity and duration to warrant a diagnosis. The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as being: "... depressed, sad, hopeless, discouraged, or 'down in the dumps'."
Depression is the mistress, the secret lover of happiness. It is the yang of ying. The darkness to the light.