1) A disorder characterized by persistent developmentally inappropriate inattention and impulsivity.

2) A syndrome of disordered learning and disruptive behavior that is not caused by any serious underlying physical or mental disorder and that has several subtypes characterized primarily by symptoms of inattentiveness or primarily by symptoms of hyperactivity and impulsive behavior (as in speaking out of turn) or by the significant expression of all three -- abbreviation ADD; called also minimal brain dysfunction.