Monotonous function or monotonic function

"Monotonic" or "monotone", but not "monotonous" (boring).

That would be monotonic function. Monotonic is always used in relation to the function you are talking about.

http://mathworld.wolfram.com/MonotonicFunction.html

Monotonic describes something this is unchanged or altered, such as the function in maths whereas Monotonous describes something lacking in variety and is usually used in reference to tone.

monotonic for sure is to be used. Monotonic functions are those functions which are either increasing or decreasing. They are such that for each specific value of x there is a unique y(value of function) which does not repeat for any other x.