Depression is when you have at least 5 or more of the following symptoms.
1) Depressed mood most of the day, nearly every day. (In children and Adolescents it can be an irritable mood).
2) Markedly diminshed interest or pleasure in all, or almost all, activities, most of the day, almost every day.
3) Significant weight loss when not dieting or weight gain. A decrease or increase in appetite nearly every day.
4) Insomnia or hypersomnia nearly every day.
5) Psychomotor agitation or retardation nearly every day.
6) Fatigue or loss of energy nearly every day.
7) Feelings of worthlessness, or excessive and inappropriate guilt nearly every day.
8) Diminished ability to think or concentrate, or indecisiveness nearly every day.
9) Recurrent thoughts of death, suicide with or without a specific plan.
It also must be noted that the symptoms cause clinically significant distress or impairment in social, occupational, or other areas of functioning.