monotone(having a single pitch) adjective [UK: ˈmɒ.nə.təʊn] [US: ˈmɑː.nəˌtoʊn]
monotonic(mathematics: never decreasing or increasing as its independent variable increases) adjective [UK: mənətˈɒnɪk] [US: mənətˈɑːnɪk]
monotonous(having an unvarying tone or pitch) adjective [UK: mə.ˈnɒ.tə.nəs] [US: mə.ˈnɑː.tə.nəs]She says her life is monotonous. = Ella dice que su vida es monótona.