depression (physiology) depression (physiology) in physiology and medicine, depression refers to a lowering, in particular a reduction in a specific biological variable or the functions of an organ. it is the opposite of elevation. for example, it is possible to refer to "depressed thyroid function" or to a depression of blood flow in a particular area. not to be confused with depression (mood) or major depressive disorder. further examples: depression of the central nervous system of an animal may be expressed as drowsiness or sleep, lack of coordination and unconsciousness. respiratory depression or hypoventilation.