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Assimilation is a common phonological process by which the phonetics of a speech segment becomes more like that of another segment in a word (or at a word boundary). A common example of assimilation would be "don't be silly" where the /n/ and /t/ in "don't" are assimilated to /m/ and /p/ by the following /b/, where said naturally in many accents and discourse styles ("dombe silly"). Assimilation can be synchronic being an active process in a language at a given point in time or diachronic being a historical sound change.
A related process is coarticulation where one segment influences another to produce an allophonic variation, such as vowels acquiring the feature nasal before nasal consonants when the velum opens prematurely or /b/ becoming labialised as in "boot". This article will describe both processes under the term, assimilation.
المصدر: http://en.wikipedia.org/wiki/Assimilation_(linguistics)