Source : Webster's Revised Unabridged Dictionary (1913)

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.]
   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.

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

            By his side, As in a glistering zodiac, hung the
            sword.                                --Milton.

Source : WordNet®

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