Synonyms for Flatness (same or very similar meaning)
WordNet sense 1 (the property of having little or no contrast; lacking highlights or gloss):
mat, matt, matte, lusterlessness, lustrelessnessWordNet sense 2 (inactivity; showing an unusual lack of energy):
languor, lethargy, phlegm, sluggishnessWordNet sense 3 (the property of having two dimensions):
planeness, two-dimensionalityFrom the ODE community, based on WordNetadd/edit