Definition - A tone that is used to convey lexical meaning.
Note: That means that a toneme is a type of phoneme.

Etymology -
Here's the story of how the word was coined:
"One day — …about February 1921 — he [sc. D. M. Beach] gave a lecture in the Department of Phonetics at University College, London, … in the course of which he demonstrated that each of the four so-called ‘tones’ of that language [sc. Pekingese] had ‘variants’ conditioned by the tones of syllables adjoining them in connected speech, and sometimes by other factors. The word ‘toneme’ was coined at my suggestion: it was readily accepted by Beach and the … staff of the Department."
(Source: D. Jones in the History and Meaning of the Term ‘Phoneme’ (1957), p 12)

Oxford English Dictionary -
Its first citation is from 1923:
"Corresponding to the phoneme is the toneme, which is a group of tones no one of which may be used in the same position as any other."
(D. M. Beach, Phonetics of Pekingese (London Univ. thesis) ii. 9 )

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