Absent-mindedness is the habit, or the state, of forgetting things because your thoughts are elsewhere. It is the noun: not “he is absent-minded,” but “his absent-mindedness made him miss the stop.” People treat it as a small human failing, often linked with being busy or living in your head. Though actually, everyone has a bit of it when they are tired or worried; we only name it when it shows — the lost keys, the phone left on the bus. By itself it does not mean a disease. It is a slightly long, slightly bookish noun; in speech many people just say someone is absent-minded and leave it there.