An ELT Glossary : Assimilation

• Definition : The way that, in connected speech, a speech sound changes quality and becomes more like another sound which precedes or follows it so that the two sounds become easier to pronounce together. 

• Examples :  

         - In "fruit buns" the alveolar /t/ will change to a bilabial /p/ in order to have the same                                 place of articulation as the following bilabial /b/ : /fru:p bʌnz/
         - Similarly, in "good cook" the alveolar /d/ changes to a velar /g/ before the velar /k/ :   /gʊg kʊk/

• In English, assimilation is nearly always regressive (ie the second sound causes the first to change).