monotone

Short definition from Wiktionary

Adjective

1. 1
   Having a single unvaried pitch.
2. 2
   Of a function: that is always nonincreasing or nondecreasing on an interval.
3. 3
   Synonym of monochrome.

Noun

1. 1
   A single unvaried tone of speech or a sound.
2. 2
   A piece of writing in one strain throughout.

Verb

1. 1
   To speak in a monotone.