A single unvaried tone of speech or a sound.

“When Tima felt like her parents were treating her like a servant, she would speak in monotone and act as though she were a robot.”

A piece of writing in one strain throughout.

To speak in a monotone.

(of speech or a sound) Having a single unvaried pitch.

Being, or having the salient properties of, a monotone function.

“The function f(x):=x^3 is monotone on \R, while g(x):=x^2 is not.”