Depression (psychology), mental illness in which a person experiences deep, unshakable sadness and diminished interest in nearly all activities. People also use the term depression to describe the temporary sadness, loneliness, or blues that everyone feels from time to time. In contrast to normal sadness, severe depression, also called major depression, can dramatically impair a person’s ability to function in social situations and at work. People with major depression often have feelings of despair, hopelessness, and worthlessness, as well ....