A low, soft, sibilant voice or utterance, which can be
   heard only by those near at hand; voice or utterance that employs only
   breath sound without tone, friction against the edges of the vocal
   cords and arytenoid cartilages taking the place of the vibration of the
   cords that produces tone; sometimes, in a limited sense, the sound
   produced by such friction as distinguished from breath sound made by
   friction against parts of the mouth. See Voice, n., 2, and Guide to
   Pronunciation, // 5, 153, 154.