Flat, uniform (4)
I believe the answer is:
even
I believe this is a double definition.
'flat' is the first definition.
(level)
'uniform' is the second definition.
(both can mean equal or level)
(Other definitions for even that I've seen before include "That's not odd" , "Flat - yet" , "Pantomime" , "Like 2,4,6, ..." , "Flat, level" .)