(zōʹdē-ăk´) noun. 1. a. Astronomy. A band of the celestial sphere extending about 8° to either side of the ecliptic that represents the path of the principal planets, the moon, and the sun. b. In astrology, this band divided into 12 equal parts called signs, each 30° wide, bearing the name of a constellation for which it was originally named but with which it no longer coincides owing to the precession of the equinoxes. c. A diagram or figure representing the zodiac. 2. A complete circuit; a circle, zodiak.