the quality of having a level surface without raised areas or indentations

the flatness of the external surface

make sure that the paper is of uniform flatness

lack of emotion or enthusiasm; dullness

neither the editing nor the score do much to help the film's emotional flatness

there's no escaping the flatness of the characters

loss of effervescence in a sparkling drink

the quality of a musical sound being below normal pitch

the flatness of the vocals