The Collaborative International Dictionary of English v.0.48:

Twitter \Twit"ter\, n. [From Twit.]
   One who twits, or reproaches; an upbraider.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twitter \Twit"ter\, v. i. [imp. & p. p. Twittered; p. pr. &
   vb. n. Twittering.] [OE. twiteren; of imitative origin; cf.
   G. zwitschern, OHG. zwizzir[=o]n, D. kwetteren, Sw. qvittra,
   Dan. quiddre. Cf. Titter.]
   1. To make a succession of small, tremulous, intermitted
      noises.
      [1913 Webster]

            The swallow twittering from the straw-built shed.
                                                  --Gray.
      [1913 Webster]

   2. To make the sound of a half-suppressed laugh; to titter;
      to giggle. --J. Fletcher.
      [1913 Webster]

   3. [Perhaps influenced by twitch.] To have a slight trembling
      of the nerves; to be excited or agitated.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twitter \Twit"ter\, v. t.
   To utter with a twitter. --Cowper.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twitter \Twit"ter\, n.
   1. The act of twittering; a small, tremulous, intermitted
      noise, as that made by a swallow.
      [1913 Webster]

   2. A half-suppressed laugh; a fit of laughter partially
      restrained; a titter; a giggle. --Hudibras.
      [1913 Webster]

   3. A slight trembling or agitation of the nerves.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twitter
    n 1: a series of chirps [syn: chirrup, twitter]
    v 1: make high-pitched sounds, as of birds [syn: chitter,
         twitter]

Moby Thesaurus II by Grady Ward, 1.0:

261 Moby Thesaurus words for "twitter":
   ado, agitation, anthem, babble, ballad, banterer, bluster, bob,
   bobbery, bobble, boil, boiling, bother, bounce, brouhaha, bump,
   bustle, cackle, call, carol, caw, chaffer, chant, chat, chatter,
   cheep, chip, chipper, chirk, chirm, chirp, chirping, chirr,
   chirrup, chitter, choir, chorus, chuck, churn, clack, cluck,
   cock-a-doodle-doo, commotion, conturbation, coo, croak, cronk,
   croon, crow, cuckoo, descant, didder, discomposure, disorder,
   disquiet, disquietude, disturbance, dither, dithers, do-re-mi,
   drum, ebullition, embroilment, excitement, falter, ferment,
   fermentation, fever, feverishness, fidget, fidgetiness, fidgets,
   fidgety, flap, flip out, flurry, fluster, flusteration, flustered,
   flustration, flutter, flutteration, foment, foofaraw, freak out on,
   fret, fume, fuss, gab, gabble, gaggle, gas, get high on, giggle,
   glow, go pitapat, gobble, goosey, gossip, grimace, guggle,
   have an ague, have the fidgets, have the shakes, heave, heaving,
   high-strung, honk, hoo, hoot, hubbub, hum, hurly-burly, hustle,
   hymn, inquietude, intonate, intone, jactitate, jar, jaw, jerk, jig,
   jigget, jiggle, jitters, jittery, jog, joggle, jollyer, jolt,
   josher, jostle, jounce, jump, jumpiness, jumpy, kidder, lather,
   lilt, maelstrom, malaise, minstrel, moil, nerviness, nervosity,
   nervousness, nervy, palpitate, palpitation, pant, panting, peep,
   persifleur, perturbation, pip, pipe, pitapat, pitter-patter,
   pother, prate, prattle, psalm, pucker, quack, quake, quaking,
   quaver, quavering, quiver, quivering, ragger, rattle, restlessness,
   ribber, rictus, roaster, roil, roll, roulade, rout, row, run on,
   scold, seethe, seething, serenade, shake, shakes, shaking, shiver,
   shivers, shock, shudder, simper, sing, sing in chorus, snicker,
   snigger, sol-fa, solmizate, spooky, squawk, squirm, stew, stir,
   swell, swell with emotion, swirl, swivet, thrill, thrill to, throb,
   throbbing, tic, tingle, tingle with excitement, titter, tizzy,
   to-do, toss, toss and turn, tremble, trembling, tremolo, tremor,
   trepidation, trepidity, trill, trilling, troll, tumble, tumult,
   tumultuation, turbidity, turbulence, turmoil, turn on to, tweedle,
   tweedledee, tweet, twist and turn, twit, twitch, twitteration,
   twittery, unease, unrest, unrestful, upset, vibrate, vocalize,
   warble, warbling, whirl, whistle, wiggle, wobble, wriggle, writhe,
   yodel



The Free On-line Dictionary of Computing (30 December 2018):

Twitter

    A free Internet service for posting short
   messages, known as "tweets", via a central server, which are
   then sent to all users who have chosen to follow you or to a
   specific user.  A variety of client programs are available
   in addition to the website.  Launched in about 2008.

   Twitter home (http://twitter.com/).

   (2009-06-10)