1.NounAn abstract phonological unit that represents collectively the dialectal variants of a phoneme.
"The transcription /oː‖əʊ‖oʊ/ or {oː} represents the diaphoneme descended from the ancestral Old English phoneme /oː/, which is preserved in Irish English as [oː] but has become [əʊ] in Received Pronunciation and as [oʊ] in General American."