The Collaborative International Dictionary of English v.0.48:

Carol \Car"ol\, n. [OF. carole a kind of dance wherein many
   dance together, fr. caroler to dance; perh. from Celtic; cf.
   Armor. koroll, n., korolla, korolli, v., Ir. car music, turn,
   circular motion, also L. choraula a flute player, charus a
   dance, chorus, choir.]
   1. A round dance. [Obs.] --Chaucer.
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   2. A song of joy, exultation, or mirth; a lay.
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            The costly feast, the carol, and the dance. --Dryden
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            It was the carol of a bird.           --Byron.
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   3. A song of praise of devotion; as, a Christmas or Easter
      carol.
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            Heard a carol, mournful, holy.        --Tennyson.
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            In the darkness sing your carol of high praise.
                                                  --Keble.
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   4. Joyful music, as of a song.
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            I heard the bells on Christmans Day
            Their old, familiar carol play.       --Longfellow.
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The Collaborative International Dictionary of English v.0.48:

Carol \Car"ol\, v. t. [imp. & p. p. Caroled, or Carolled; p.
   pr. & vb. n. Caroling, or Carolling.]
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   1. To praise or celebrate in song.
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            The Shepherds at their festivals
            Carol her goodness.                   --Milton.
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   2. To sing, especially with joyful notes.
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            Hovering swans . . . carol sounds harmonious.
                                                  --Prior.
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The Collaborative International Dictionary of English v.0.48:

Carol \Car"ol\, v. i.
   To sing; esp. to sing joyfully; to warble.
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         And carol of love's high praise.         --Spenser.
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         The gray linnets carol from the hill.    --Beattie.
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The Collaborative International Dictionary of English v.0.48:

Carol \Car"ol\, Carrol \Car"rol\, n. [OF. carole a sort of
   circular space, or carol.] (Arch.)
   A small closet or inclosure built against a window on the
   inner side, to sit in for study. The word was used as late as
   the 16th century. The term carrel, of the same has largely
   superseded its use.
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         A bay window may thus be called a carol. --Parker.
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WordNet (r) 3.0 (2006):

carol
    n 1: joyful religious song celebrating the birth of Christ [syn:
         carol, Christmas carol]
    2: a joyful song (usually celebrating the birth of Christ)
    v 1: sing carols; "They went caroling on Christmas Day"

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "carol":
   Brautlied, Christmas carol, Kunstlied, Liebeslied, Volkslied, alba,
   anthem, art song, aubade, ballad, ballade, ballata, barcarole,
   blues, blues song, boat song, bridal hymn, brindisi, cackle, call,
   calypso, canso, canticle, canzone, canzonet, canzonetta, caper,
   caracole, cavatina, caw, chanson, chant, chantey, chatter, cheep,
   chirk, chirp, chirr, chirrup, chitter, choir, chorus, chuck, clack,
   clap hands, cluck, cock-a-doodle-doo, coo, croak, cronk, croon,
   croon song, crow, cuckoo, dance, delight, descant, dirge, ditty,
   do-re-mi, drinking song, drum, epithalamium, exult, folk song,
   frisk, frolic, gabble, gaggle, gambol, glory, gobble, guggle, honk,
   hoo, hoot, hum, hymeneal, hymn, intonate, intone, joy, jubilate,
   lay, lied, lilt, love song, love-lilt, matin, minstrel,
   minstrel song, minstrelsy, national anthem, peep, pip, pipe,
   prothalamium, psalm, quack, quaver, rejoice, revel, roll, rollick,
   romp, roulade, scold, serena, serenade, serenata, shake, sing,
   sing in chorus, skip, skip for joy, sol-fa, solmizate, song,
   squawk, theme song, torch song, tremolo, trill, troll, tweedle,
   tweedledee, tweet, twit, twitter, vocalize, war song, warble,
   wedding song, whistle, yodel