In a certain interval, if the function f(x) only increases (monotonically increases) or decreases (monotonically decreases) with the increase of the independent variable x, then the function is monotonic in this interval. If the function f(x) only increases (increasing function) or decreases (subtracts) with the increase of the independent variable x in the whole definition domain, the function is monotonous.

From the image, from left to right along the X axis, the function only presents an upward or downward trend, which is monotonous, otherwise it is not monotonous.