Definitions

1. adjective

having or characterized by a single note or pitch; lacking variation in pitch or tone

“The singer’s monotone voice made the song feel unengaging.”

2. noun

a sound or voice that is unvaried in pitch or tone

“The monotone of the alarm clock woke me up in the morning.”

3. adverb

in a monotone; in a voice or manner lacking variation in pitch or tone

“She sang the song monotone, which was quite unusual for her.”