define disease
disease. noun. 1  An abnormal condition of a part, organ, or system of an organism resulting from various causes, such as infection, inflammation, environmental factors, or genetic defect, and characterized by an identifiable group of signs, symptoms, or both.isease. 1  any departure from health; illness in general. 2  a particular destructive process in an organ or organism, with a specific cause and characteristic symptoms; specif., an illness; ailment. 3  any harmful or destructive condition, as of society.