flatness (Noun) — The property of having two dimensions.
flatness (Noun) — A want of animation or brilliance. ex. "the almost self-conscious flatness of Hemingway's style"
flatness (Noun) — A deficiency in flavour. ex. "it needed lemon juice to sharpen the flatness of the dried lentils"
flatness (Noun) — The property of having little or no contrast; lacking highlights or gloss.
flatness (Noun) — Inactivity; showing an unusual lack of energy. ex. "the general appearance of flatness alarmed his friends"