In phonetics and historical linguistics, fusion, or coalescence, is a sound change where two or more segments with distinctive features merge into a single segment. This can occur both on consonants and in vowels. A word like educate is one that may exhibit fusion, e.g. /?dj?ke?t/ or /'?dke?t/. A merger between two segments can also occur between word boundaries, an example being the phrase got ya being pronounced like gotcha /t/. Most cases of fusion lead to allophonic variation, though some sequences of segments may lead to wholly distinct phonemes.
A common form of fusion is found in the development of nasal vowels, which frequently become phonemic when final nasal consonants are lost from a language. This occurred in French and Portuguese. Compare the French words un vin blanc [oe? v bl] "a white wine" with their English cognates, one, wine, blank, which retain the n's.
Vowel coalescence is extremely common. The resulting vowel is often long, and either between the two original vowels in vowel space, as in [ai] -> [e:] -> [e] and [au] -> [o:] -> [o] in French (compare English day [de?] and law [l?:]), in Hindi (with [?:], [?:]), and in some varieties of Arabic; or combines features of the vowels, as in [ui] -> [y:] -> [y] and [oi] -> [ø:] -> [ø].
Compensatory lengthening may be considered an extreme form of fusion.
Historically, the alveolar plosives and fricatives have fused with /j/, in a process referred to as yod coalescence. Words like nature and omission have had such consonant clusters, being pronounced like /na:'tiu?r/ and /?'misj?n/. Words ending in the Latin-derived suffixes -tion and -sion, such as fiction and mission, are examples that exhibit yod coalescence.
This sound change was not, however, distributed evenly. Words like module may be realised as either /'m?dju:l/ or /'m?d?u:l/. Words that did not experience universal yod coalescence, are always realised as two segments in accents like Received Pronunciation. Most other dialects do pronounce them as one segment, however, like American English.
Words with primary stress on a syllable with such a cluster did not experience coalescence either. Examples include tune /tju:n/ and assume . Some dialects exhibit coalescence in these cases, where some coalesce only /tj/ and /dj/, while others also coalesce /sj/ and /zj. In General American, /j/ elides entirely when proceding alveolar consonants, in a process called yod dropping. The previous examples end up as /tu:n/ and . Words that have already coalesced are not affect by this.
Australian English exhibits yod coalescence to an extreme degree, even when the cluster is in a stressed syllable, though there is some sociolectal variation. In an accent with full yod coalescence, tune and assume are pronounced like /t?u:n/ and /?'?u:m/. This can result in homophony between previously distinct words, as between tuner and tuna, which are both pronounced /t?u:n?/.
Most Romance languages have coalesced sequences of consonant clusters proceded by /j/. Sequences of plosives proceded by /j/ most often became affricates, often being intermediary stages to other manners of articulation. Sonorants in such a sequence (except bilabial consonants) mostly became palatalized.
During the development of Ancient Greek from Proto-Greek, the labiovelar [k?], [k], and  became [p], [p?], and [b]. Although the labiovelars were already a single consonant, they had two places of articulation, a velar articulation and labial secondary articulation ([?]). However, the development of labiovelars varies from dialect to dialect, and some may have become dental instead. An example is the word boûs "cow" from Proto-Greek *g?ous.
A vowel coalescence from Ancient Greek to Koine Greek fused many diphthongs, especially those including /i?/. E.g. /ai?/ > /e/; /a:i?/ > /a/; /?:i?/ and /oi?/ > /i/ and /?:i?/ > /o/.
In Norwegian and Swedish, this process occurs whenever the phoneme /?/ is followed by an alveolar consonant. The articulation of the resulting fusion becomes retroflex. Examples include the Norwegian bart [b] and Swedish nu:? [nu:?]. This even occurs across word boundaries, as in the sentence "går det bra?" becoming /?o: br?:/.
This process will continue for as long as there are more alveolar consonants, though when this amount exceeds four, people usually try to break it up or shorten it, usually by replacing /?/ with /s/, or eliding /d/. An extreme example of this would be the word ordensstraff /?.?.?f:/, having six retroflex consonants in a row.
In colloquial Norwegian, the sequence /rt/ may even coalesce over non-alveolar phonemes, changing their place of articulation to retroflex, even if /r/ normally wouldn't trigger it. Examples include sterkt /stæ?kt/ [stæ?:], skarpt /skpt/ [sk:], verktøy /?æ?k.toe/ [?æ?.?oe] and varmt /mt/ [?]. This process does not occur across word boundaries, e.g. sterk tann is pronounced /stæ?k t?n:/ and not */stæn:/
In dialects where /r/ is articulated uvularly, this process invariably takes place on idiolectal level. For example, /r?:rt/ may be realised as [:?t] or [:?]. This may appear in regions where /r/ has recently become uvular.
In Malay, the final consonant of the prefix /m?N-/ (where N stands for a "placeless nasal", i.e. a nasal with no specified place of articulation) coalesces with a voiceless stop at the beginning of the root to which the prefix is attached. The resulting sound is a nasal that has the place of articulation of the root-initial consonant. For example:
Vowel coalescence occurs in Owari Japanese. The Diphthongs /ai/ and /ae/ change to [æ:], /oi/ and /oe/ change to [ø:] and /ui/ changes to [y:]. E.g. /raineN/ > [ræ:nen], ? /koi/ > [kø:], /atsui/ > [atsy:~at?:y:]. Younger speakers may vary between Standard Japanese diphthongs and dialectal monophthongs.