👉 Monotonicity is a property of a function that describes how it behaves with respect to increasing or decreasing values. If a function f(x) is monotonically increasing (or decreasing), then as x increases, f(x) also increases (decreases) by the same amount. Conversely, if f(x) is monotonically decreasing (increasing), then as x decreases, f(x) also decreases by the same amount.