The Collaborative International Dictionary of English v.0.48:

Astrology \As*trol"o*gy\ ([a^]s*tr[o^]l"[-o]*j[y^]), n. [F.
   astrologie, L. astrologia, fr. Gr. 'astrologi`a, fr.
   'astrolo`gos astronomer, astrologer; 'asth`r star + lo`gos
   discourse, le`gein to speak. See Star.]
   In its etymological signification, the science of the stars;
   among the ancients, synonymous with astronomy; subsequently,
   the art of judging of the influences of the stars upon human
   affairs, and of foretelling events by their position and
   aspects.
   [1913 Webster]

   Note: Astrology was much in vogue during the Middle Ages, and
         became the parent of modern astronomy, as alchemy did
         of chemistry. It was divided into two kinds: judicial
         astrology, which assumed to foretell the fate and acts
         of nations and individuals, and natural astrology,
         which undertook to predict events of inanimate nature,
         such as changes of the weather, etc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

astrology
    n 1: a pseudoscience claiming divination by the positions of the
         planets and sun and moon [syn: astrology, star
         divination]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "astrology":
   Friday, Friday the thirteenth, appointed lot, aspect,
   astral influences, astrodiagnosis, astromancy, augury,
   book of fate, clairvoyance, constellation, crystal ball,
   crystal gazing, cup, destination, destiny, dies funestis,
   divination, divining, doom, end, fatality, fate, foredoom, fortune,
   fortunetelling, future, genethliac astrology, genethliacism,
   genethliacs, genethlialogy, haruspication, haruspicy, horoscope,
   horoscopy, house, ides of March, inevitability, kismet, lot,
   mansion, mantic, mantology, moira, mundane astrology,
   mundane house, nativity, natural astrology, palm-reading,
   palmistry, planetary house, planets, portion, pythonism, sorcery,
   stargazing, stars, unlucky day, weird, wheel of fortune,
   will of Heaven, zodiac