The Collaborative International Dictionary of English v.0.48:

Whisper \Whis"per\, v. i. [imp. & p. p. Whispered; p. pr. &
   vb. n. Whispering.] [AS. hwisprian; akin to G. wispern,
   wispeln, OHG. hwispal?n, Icel. hv[imac]skra, Sw. hviska, Dan.
   hviske; of imitative origin. Cf. Whistle.]
   [1913 Webster]
   1. To speak softly, or under the breath, so as to be heard
      only by one near at hand; to utter words without sonant
      breath; to talk without that vibration in the larynx which
      gives sonorous, or vocal, sound. See Whisper, n.
      [1913 Webster]

   2. To make a low, sibilant sound or noise.
      [1913 Webster]

            The hollow, whispering breeze.        --Thomson.
      [1913 Webster]

   3. To speak with suspicion, or timorous caution; to converse
      in whispers, as in secret plotting.
      [1913 Webster]

            All that hate me whisper together against me. --Ps.
                                                  xli. 7.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whisper \Whis"per\, n.
   [1913 Webster]
   1. 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, [sect][sect]
      5, 153, 154.
      [1913 Webster]

            The inward voice or whisper can not give a tone.
                                                  --Bacon.
      [1913 Webster]

            Soft whispers through the assembly went. --Dryden.
      [1913 Webster]

   2. A cautious or timorous speech. --South.
      [1913 Webster]

   3. Something communicated in secret or by whispering; a
      suggestion or insinuation.
      [1913 Webster]

   4. A low, sibilant sound. "The whispers of the leaves."
      --Tennyson.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whisper \Whis"per\, v. t.
   [1913 Webster]
   1. To utter in a low and nonvocal tone; to say under the
      breath; hence, to mention privately and confidentially, or
      in a whisper.
      [1913 Webster]

            They might buzz and whisper it one to another.
                                                  --Bentley.
      [1913 Webster]

   2. To address in a whisper, or low voice. [Archaic]
      [1913 Webster]

            And whisper one another in the ear.   --Shak.
      [1913 Webster]

            Where gentlest breezes whisper souls distressed.
                                                  --Keble.
      [1913 Webster]

   3. To prompt secretly or cautiously; to inform privately.
      [Obs.] "He came to whisper Wolsey." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

whisper
    n 1: speaking softly without vibration of the vocal cords [syn:
         whisper, whispering, susurration, voicelessness]
    2: a light noise, like the noise of silk clothing or leaves
       blowing in the wind [syn: rustle, rustling, whisper,
       whispering]
    v 1: speak softly; in a low voice [ant: shout]

Moby Thesaurus II by Grady Ward, 1.0:

246 Moby Thesaurus words for "whisper":
   advice, alert, alerting, articulate, aspirate, aspiration, bark,
   bated breath, bawl, bellow, blare, blat, blubber, boom, bray,
   breath, breathe, breathy voice, broad hint, bruit, brush, buzz,
   cackle, canard, caress, caution, chant, chime, chirp, chorus, clue,
   come out with, common talk, communicate, confide, confide to,
   contact, convey, coo, crow, cry, cue, cutaneous sense, dash,
   deliver, disclose, drawl, drone, droning, emit,
   entrust with information, enunciate, exclaim, exhalation, express,
   feel, feeling, fingertip caress, fizz, fizzle, flick, fling off,
   flute, flying rumor, formulate, gabble, gasp, gentle hint, gesture,
   gibber, gibbering, give, give confidential information,
   give expression, give out with, give tongue, give utterance,
   give voice, glance, glimmer, glimmering, grapevine, graze, groan,
   growl, grunt, hand-mindedness, hearsay, hint, hiss, howl,
   idea afloat, impart, implication, index, indication, inkling,
   innuendo, insinuation, intimation, jabber, jibber, keen, kick,
   kiss, lambency, lap, latrine rumor, let in on, let next to,
   let out, lick, light touch, lilt, lip, little voice, look,
   low voice, maffle, maunder, maundering, mention privately, moan,
   monition, mouth, mouthing, mumble, mumbling, murmur, murmuration,
   murmuring, mussitate, mutter, muttering, news stirring, nod, nudge,
   office, on-dit, out with, pant, passing word, phonate, phrase,
   pipe, pointer, pour forth, present, prompt, pronounce, put forth,
   put hep, put in words, put next to, raise, report, roar, roorback,
   rub, rumble, rumor, say, scent, scream, screech, scuttlebutt,
   sense of touch, set forth, shade, shriek, sibilate, sigh, sign,
   signal, sing, sizzle, snap, snarl, snort, sob, soft voice, sough,
   sound, soupcon, speak incoherently, splutter, spoor, sputter,
   squall, squawk, squeal, stage whisper, steer, still small voice,
   stroke, suggestion, suspicion, susurrate, susurration, susurrus,
   swish, symptom, tactile sense, taction, take aside, talk, tap,
   tell, tell confidentially, telltale, tentative poke, throw off,
   thunder, tinge, tip, tip off, tip-off, touch, town talk, trace,
   track, trumpet, twang, unconfirmed report, underbreath, undertone,
   utter, verbalize, vocalize, voice, wail, warble, warning, wheeze,
   whiff, whine, whispering, whistle, whiz, whoosh, wink, word, yap,
   yawp, yell, yelp