(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 