Depression is a mood disorder in which a persistent feeling of sadness lasts at least two weeks; during which there is a depressed mood (feeling: sad, hopeless, helpless, discouraged), or the loss of interest, or pleasure in nearly all activities.
There can also be changes in appetite or weight, sleep, and psychomotor activities (physical movement, coordination), decreased energy, feelings of worthlessness, or guilt, difficult thinking-concentrating, or making decisions, or recurrent thoughts of death, or suicidal ideation.