flatness n. + Add to list US //ˈfɫætnəs// UK //flˈætnəs// flat·ness SynonymsEtymology Synonyms languorlethargylusterlessnessmatmattmatteplanenesssluggishnesstwo dimensionalityOriginMEflatnesse→ENflatness From Middle English flatnesse, equivalent to flat + -ness.