Monomorphism is the opposite of polymorphism. That is, a function is polymorphic if it works for several different types - and thus, a function is monomorphic if it works only for one type.
As an example,
map is polymorphic. It's type is simply
map :: (a -> b) -> [a] -> [b]
However, the function
foo :: (Int -> Int) -> [Int] -> [Int] foo = map
performs an identical operation to
map (as is evident from the second line), but has a monomorphic type; it will only accept lists of
Int and functions over them.
Perhaps you were looking for monomorphism restriction?