assibilation

from The Century Dictionary.

• noun The act of making sibilant; specifically, in philology, the change of a dental or guttural (or a labial) mute into a sibilant (s, z, sh, zh, ch = tsh, j = dzh), or into a sound approaching that of a sibilant, as for instance a palatal.

from the GNU version of the Collaborative International Dictionary of English.

• noun Change of a non-sibilant letter to a sibilant, as of -tion to -shun, duke to ditch.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun linguistics A sound change resulting in a sibilant consonant, commonly the final phase of palatalization.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun pronunciation with a sibilant (hissing or whistling) sound
• noun the development of a consonant phoneme into a sibilant