Definition of "monotone"

The prominence of the syllables is more monotone than in English, the intonation of the latter having a larger variation of stressed and unstressed syllables.

1940, Asiatic Society (Calcutta, Royal Asiatic Society of Bengal, India), Journal of the Asiatic Society, page 95

In the formal register, such variation is reduced and the talk has a more monotone, business-like quality.

1998, Roger W. Shuy, “Bureaucratic Language in Government and Business”, in Research on Telephone vs. In-Person Administrative Hearings, Georgetown University Press, page 76

It is no very difficult matter to be loud in a high tone of voice; but to be loud and forcible in a low tone, requires great practice and management; this, however, may be facilitated by pronouncing forcibly at firſt in a low monotone; a monotone, though in a low key, and without force, is much more ſonorous and audible than when the voice ſlides up and down at almoſt every word, as it muſt do to be various.

1799, John Walker, Elements of Elocution, Cooper and Wilson, page 309

There is a water-break formed by a small terrace of rock in mid-stream, and purling with a hollow, delicious monotone—an island of pebbles is above, with here and there smaller ones near the "forks."

1846 October, Alfred B[illings] Street, “A Day’s Hunting about the Mongaup”, in George R[ex] Graham, editor, Graham’s American Monthly Magazine of Literature and Art, volume XXIX, number 4, Philadelphia, Pa.: George R. Graham & Co., […], page 190