Definition of cyanosis a bluish color of skin and mucous membranes, especially in lips, nail beds, and malar eminences, caused by increased amount of reduced hemoglobin(Hb)or abnormal Hb derivatives in small blood vesselsDefinition of cyanosis A bluish color of skin and mucous membranes, especially in lips, nail beds, and malar eminences, caused by increased amount of reduced hemoglobin(Hb) or abnormal Hb derivatives in small blood vessels