What is the property of monotonicity of derivative function?

What is the property of monotonicity of derivative function?

For example, for the derivative f '(x) of function f (x), if f' (x) is always greater than or equal to zero (and not always zero) in a certain domain, then f (x) increases monotonically in this domain; if f '(x) is always less than or equal to zero (and not always zero) in a certain domain, then f (x) decreases monotonically in this domain