noun (n)

• the development of a consonant phoneme into a sibilant(noun.process)

  source: wordnet30
• pronunciation with a sibilant (hissing or whistling) sound(noun.communication)

  Synonym:
  sibilation
  

  source: wordnet30
• Change of a non- sibilant letter to a sibilant, as of -tion to -shun, duke to ditch.(noun)

  source: webster1913