1 insert a sibilant sound before or after (another sound)
2 change into a sibilant; "In the syllable /si/, the /s/ sibilates in Japanese"
In linguistics, assibilation is the term for a sound change resulting in a sibilant consonant. It is commonly the final phase of palatalization.
The word "assibilation" itself contains an example of the phenomenon, being pronounced /əsɪbɪleɪʃən/. The classical Latin tio was pronounced as /tio/ (for example, assibilatio was prounounced /asːibilatio/ and attentio /atːentio/). However, in Vulgar Latin it assibilated to /tsio/, and this can still be seen in Italic: attenzione. In French lenition gave /sjə/, which in English then palatalized to the /ʃə/.
For another example, in the history of Finnish, /ti/ changed to /si/. The alternation can be seen in dialectal and inflected word-forms: kielti vs. kielsi "s/he denied"; vesi "water", vetenä "as water".
Assibilation can also occur outside of palatalization. One example is the replacement of th with s or z characterizing a French accent of English.
assibilate in Breton: C'hwitelladur (yezhoniezh)
assibilate in Spanish: Asibilación
assibilate in French: Assibilation
assibilate in Croatian: Sibilarizacija
assibilate in Low German: Assibilatschoon
assibilate in Polish: Asybilacja
assibilate in Serbian: Сибиларизација