Source : Webster's Revised Unabridged Dictionary (1913)

Sibilant \Sib"i*lant\, a. [L. sibilans, -antis, p. pr. of
   sibilare to hiss: cf. F. sibilant.]
   Making a hissing sound; uttered with a hissing sound;
   hissing; as, s, z, sh, and zh, are sibilant elementary
   sounds. -- n. A sibiliant letter.

Source : WordNet®

sibilant
     adj : of speech sounds produced by forcing air through a
           constricted passage (as `f', `s', `z', or `th' in both
           `thin' and `then') [syn: {fricative}, {spirant}]

sibilant
     n : a consonant characterized by a hissing sound (like s or sh)
         [syn: {sibilant consonant}]