a vocal utterance or series of speech sounds in one unvaried tone.

a single tone without harmony or variation in pitch.

recitation or singing of words in such a tone.

a person who is unable to discriminate between or to reproduce differences in musical pitch, esp. in singing.

sameness of tone or color, sometimes to a boring degree.

monotonous.

consisting of or characterized by a uniform tone of one color

a monotone drape. Compare monochromatic defs. 1, 2

Mathematics . monotonic ( def. 2 ) .

noun

humdrum , monotonousness , sameness