How do you define depression?

Group of Symptoms. Clinical depression is a collection of symptoms - something we call a syndrome - that includes a persistent and pervasive period of sadness and/or a lack of joy, usually associated with feelings of excessive guilt or apathy, difficulties getting motivated, poor concentration and energy, disrupted sleep, feelings of sluggishness or restlessness, social withdrawal, and/or thoughts of death.
See below. Daily depressed mood, lack of enjoyment in hobbies, increase or decrease in sleep, lack of interest in sex, irritability, lack of focus or concentration at school or work. Thoughts of self harm can also be a telltale sign. Depression is different for everyone. If you are unhappy and it's not improving, see your physician or a counselor.