NOUN (2)

1. a belt-shaped region in the heavens on either side to the ecliptic; divided into 12 constellations or signs for astrological purposes;

2. (astrology) a circular diagram representing the 12 zodiacal constellations and showing their signs;

The Collaborative International Dictionary of English v.0.48:

Zodiac \Zo"di*ac\, n. [F. zodiaque (cf. It. zodiaco), fr. L.
   zodiacus, Gr. ? (sc. ?), fr. ?, dim. of zw^,on an animal,
   akin to ? living, ? to live.]
   [1913 Webster]
   1. (Astron.)
      (a) An imaginary belt in the heavens, 16[deg] or 18[deg]
          broad, in the middle of which is the ecliptic, or
          sun's path. It comprises the twelve constellations,
          which one constituted, and from which were named, the
          twelve signs of the zodiac.
      (b) A figure representing the signs, symbols, and
          constellations of the zodiac.
          [1913 Webster]

   2. A girdle; a belt. [Poetic & R.]
      [1913 Webster]

            By his side,
            As in a glistering zodiac, hung the sword. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

zodiac
    n 1: a belt-shaped region in the heavens on either side to the
         ecliptic; divided into 12 constellations or signs for
         astrological purposes
    2: (astrology) a circular diagram representing the 12 zodiacal
       constellations and showing their signs

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "zodiac":
   anklet, aphelion, apogee, armlet, aspect, astrodiagnosis,
   astrology, astromancy, astronomical longitude, autumnal equinox,
   band, belt, bracelet, celestial equator, celestial longitude,
   celestial meridian, cincture, cingulum, circle, collar, collarband,
   colures, earring, ecliptic, equator, equinoctial,
   equinoctial circle, equinoctial colure, equinox, fascia, fillet,
   finger ring, galactic longitude, genethliac astrology,
   genethliacism, genethliacs, genethlialogy, geocentric longitude,
   geodetic longitude, girdle, girt, girth, great circle,
   heliocentric longitude, hoop, horoscope, horoscopy, house,
   longitude, mansion, meridian, mundane astrology, mundane house,
   nativity, natural astrology, neckband, necklace, nose ring, orbit,
   perigee, perihelion, period, planetary house, quoit, ring,
   small circle, solstitial colure, stargazing, trajectory,
   vernal equinox, wristband, wristlet, zone