In astronomy, the zodiac (Greek: ζῳδιακός, zōdiakos) is the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac. In astrology, the zodiac denotes those signs that divide the ecliptic into twelve equal zones of celestial longitude. As such, the zodiac is a celestial coordinate system, to be more precise, an ecliptic coordinate system, taking the ecliptic as the origin of latitude, and the position of the sun at vernal equinox as the origin of longitude.
It is known to have been in use by the Roman era, based on concepts inherited by Hellenistic astronomy from Babylonian astronomy of the Chaldean period (mid-1st millennium BC), which, in turn, derived from an earlier system of lists of stars along the ecliptic. The construction of the zodiac is described in Ptolemy's Almagest (2nd century AD). The term zodiac may also refer to the region of the celestial sphere encompassing the paths of the planets corresponding to the band of about eight arc degrees above and below the ecliptic. The zodiac of a given planet is the band that contains the path of that particular body; e.g., the "zodiac of the Moon" is the band of five degrees above and below the ecliptic. By extension, the "zodiac of the comets" may refer to the band encompassing most short-period comets 
The term zodiac derives from Latin zōdiacus, which in its turn comes from the Greek ζῳδιακὸς κύκλος (zōdiakos kuklos), meaning circle of animals, derived from ζώδιον (zōdion), the diminutive of ζῶον (zōon) animal. The name is motivated by the fact that half of the signs of the classical Greek zodiac are represented as animals as well as two mythological hybrids.
Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one, the term and the names of the twelve signs are today mostly associated with horoscopic astrology.
It is important to distinguish the zodiacal signs from the constellations associated with them, not only because of their drifting apart due to the precession of equinoxes but also because the physical constellations by nature of their varying shapes and forms take up varying widths of the ecliptic. Thus, Virgo takes up fully five times as much ecliptic longitude as Scorpius. The zodiacal signs, on the other hand, are an abstraction from the physical constellations designed to represent exactly one twelfth of the full circle each, or the longitude traversed by the Sun in about 30.4 days.
There have always been a number of "parazodiacal" constellations that are also touched by the paths of the planets. The MUL.APIN lists Orion, Perseus, Auriga and Andromeda. Furthermore, there are a number of constellations mythologically associated with the zodiacal ones: Piscis Austrinus, The Southern Fish, is attached to Aquarius. In classical maps, it swallows the stream poured out of Aquarius' pitcher, but perhaps it formerly just swam in it. Aquila, The Eagle, was possibly associated with the zodiac by virtue of its main star, Altair. Hydra in the Early Bronze Age marked the celestial equator and was associated with Leo, which is shown standing on the serpent on the Dendera zodiac. Corvus is the Crow or Raven mysteriously perched on the tail of Hydra. The MUL.APIN glosses Hydra as "the Snake Ningizzida, lord of the Netherworld". Ningizzida together with Dumuzi (Aries) and Pabilsag (Sagittarius) governed the household of the queen of the underworld.
Taking the current constellation boundaries as defined in 1930 by the International Astronomical Union, the ecliptic itself passes through an additional thirteenth constellation, Ophiuchus, situated between Scorpius and Sagittarius. This is already recognized in Ptolemy's Almagest. This constellation, Ophiuchus is situated between November 30 and December 17. Ophiuchus is not recognised as a general Zodiac.