What is depression officially?

Depression: Officially, "depression" is a general term that refers to the presence of depressed mood along with other symptoms. The term is sometimes used to refer to people who are depressed due to grief or other losses, and sometimes it is used to refer to episodes of severe depression that seem to arise more on their own, out of proportion to circumstances.

Non-specific word: "depression" is a very general word that can mean different things, both normal and abnormal, depending on context. It is best described as a lay word, not a specific diagnosis, meaning anything from brief, situational sadness to serious medical disorder. "major depressive disorder" (and related variations) is a diagnosis with prolonged symptoms of dysfunction and loss of pleasure feelings.