The effect of the physical articulation of a phoneme on the articulation, and therefore the sound, of an adjacent phoneme or phonemes.
Some popular linguistic theorys state that unless there is a semantically or syntatically important reason to override coarticulation, a sound can be pronounced differently depending on its neighbors.
It may be affected by the articulation of the phoneme before it, or may be affected as the mouth shapes in anticipation of the following phoneme.
A possible example in English would be the general pluralization rule. Written, this rule is more or less, "add an 's' at the end of a noun to pluralize it." However, if you listen carefully, you will find that there are actually two possible pluralizing sounds in English. The "s" sound (unvoiced), and the "z" sound (voiced).
If you pluralize a word ending in another unvoiced consonant such as the alveolar stop "t", you use the "s" sound, as in cat + s = cats.
If you pluralize a word ending in a voiced consonant (or a vowel) such as the glottal stop "g", you use the "z" sound, as in dog + s = dogz.
Go ahead...try to say dogs with the "s" sound. It'll sound and feel weird to you. You can do it, it is just less comfortable.