In phonology (linguistics), assimilation is a process of sound change in which a sound becomes similar to another sound in its immediate environment. Assimilation of sounds can happen within a word or can even cross word boundary. Usually, the sounds undergo a change in one or more features so as to assimilate to other sounds in their environment.
In phonetics and phonology, we can classify and study sound segments in terms of a set of features. Two distinct sounds can have some common features, but will never have exactly the same set of features. For example, the phoneme /p/ is bilabial, plosive and voiceless, and differs from the sound /b/ in voicelessness. You might want to read about Distinctive Features by Jacobsen et al. for a better understanding of the theory of phonological features. During assimilation, there can be a change in the place of articulation, manner of articulation, voicing, etc.
Assimilation of Place: In rapid speech, the native British English speakers would pronounce the phrase ‘ten balls’ as something like /tɛm bɔːlz/ instead of /tɛn bɔːlz/. For British English speakers, the sound /n/ changes to /m/ in the presence of the following sound /b/ in the next word. The sound /n/ is alveolar, but when followed by the bilabial sound /b/, it undergoes assimilation to become the bilabial sound /m/. Note that there is a change in only one feature, i.e. the place of articulation. The sound /n/ does not become plosive like the sound /b/.
Assimilation of Manner: In Hindi, which is an Indo-Aryan language, there are many instances where phonemes get nasalised when followed by nasal consonants or vowels. Such a change also happens during rapid speech. For example, the word for ‘work’ /kɑ:m/ is pronounced as /kɑ̃:m/. The vowel preceding the nasal sound /m/ becomes nasalised.
Assimilation of Voice: The plural morpheme –s (voiceless) in English becomes voiced when preceded by a voiced phoneme.
/dɒɡ/+ -s--> /dɒɡz/
Note that it remains voiceless when preceded by a voiceless phoneme.
/ kat/ + -s--> / kats/
The above example is also an example of progressive assimilation of voice, as the sound change is affected by the features of the sound preceding the given sound. The opposite of this is Regressive assimilation, where the sound acquires one or more features of the following sound. Nasal assimilation of Hindi vowels is an example of regressive assimilation.
Assimilation can be complete or partial. The above examples are all cases of partial assimilation. Assimilation is complete when the sound changes in all the features and becomes exactly like the sound nearby. For example, in British English the phrase ‘ten miles’ /tɛn mʌɪls/ is realised as /tɛm mʌɪls/ in rapid speech.
We can classify assimilation in one more way. When assimilation happens only at a given time era during rapid speech, we call it a synchronic change reflecting sociolinguistic events during a specific time. However, if it results in a change in the language phonology over a period of time, we call it a diachronic or historical change.