Not just sad. A major depressive episode, or major depressive disorder is not just feeling sad. It is a serious brain disease with wide ranging effect. Fortunately, it is also treatable. Symptoms include bad mood (usuly sad, but might be mostly irritable), changes in sleep, energy, appetite, weight, motivation, concentration, anhedonia, thoughts of suicide, and, in severe cases, even psychotic symptoms.
Depressive Episode. In major depressive episode, 5 or more of following symptoms present for 2 weeks & a change from normal: depressed mood; loss of interest/pleasure in activities; >5% weight change in a month; insomnia/hypersomnia; psychomotor agitation/slowing; fatigue; feelings of worthlessness/inappropriate guilt; decreased concentration or indeciciveness; recurrent thoughts of death, suicidal ideation or plan.