Definition of mean tone in US English:

mean tone

(also mean-tone, meantone)

noun

Music
  • (As two words or hyphenated.) Either of the two equal intervals of a whole tone into which a major third may be divided, used as a basis for tuning keyboard instruments before the general adoption of equal temperament.

Origin

Late 18th century; earliest use found in Thomas Young (1773–1829), physician and natural philosopher. From mean + tone.

Pronunciation

mean tone

/ˈmiːn ˌtəʊn/