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.
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.