Wiktionary
alt. (context mathematics English) A function that either never decreases or never increases as its independent variable increases. n. (context mathematics English) A function that either never decreases or never increases as its independent variable increases.
Wikipedia
In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory.
Usage examples of "monotonic function".
A psychological space is established for any set of stimuli by determining metric distances between the stimuli such that the probability that a response learned to any stimulus will generalize to any other is an invariant monotonic function of the distance between them.