(phonology) A process in English phonology whereby the clusters[dj], [tj], [sj], and [zj] become [dʒ], [tʃ], [ʃ], and [ʒ], respectively, through mutual assimilation.
1995, James M. Scobbie, “What Do We Do When Phonology is Powerful Enough to Imitate Phonetics? Comments on Zsiga”, in Bruce Connell, Amalia Arvaniti, editors, Phonology and Phonetic Evidence, Cambridge University Press, →ISBN, page 306:
Some English dialects with /dj/ and /tj/ within a morpheme have gradient amounts of affrication, from [dj] in careful speech to extreme “yod coalescence” approximating [dʒ][…].
2006, J. C. Wells, “British English Pronunciation Preferences: A Changing Scene”, in Kingsley Bolton, Braj B. Kachru, editors, World Englishes, Routledge, →ISBN, page 236:
In words such as nature this process is long complete; but there are many other words where this ‘yod coalescence’ is still variable.
2011, Paul Skandera, Peter Burleigh, A Manual of English Phonetics and Phonology, 2nd edition, Narr, →ISBN, page 149:
In the sequence would you, the female speaker uses yod coalescence: The alveolar plosive /d/ and the palatal approximant, /j/, merge to form the affricate [dʒ].
yod coalescence on Wikipedia.Wikipedia 