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