noun
a continuing sound, especially of a person's voice, that is unchanging in pitch and without intonation
adjective
having a single unvarying tone
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The professor spoke in a monotone voice throughout the entire lecture.
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I can't stand listening to that teacher, he's so monotone.
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Her presentation was so boring, she sounded like a monotone robot.
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The city skyline was a monotone of grey buildings under the overcast sky.