The Collaborative International Dictionary of English v.0.48:

Druid \Dru"id\, n. [L. Druides; of Celtic origin; cf. Ir. &
   Gael. draoi, druidh, magician, Druid, W. derwydd Druid.]
   1. One of an order of priests which in ancient times existed
      among certain branches of the Celtic race, especially
      among the Gauls and Britons.
      [1913 Webster]

   Note: The Druids superintended the affairs of religion and
         morality, and exercised judicial functions. They
         practiced divination and magic, and sacrificed human
         victims as a part of their worship. They consisted of
         three classes; the bards, the vates or prophets, and
         the Druids proper, or priests. Their most sacred rites
         were performed in the depths of oak forests or of
         caves.
         [1913 Webster]

   2. A member of a social and benevolent order, founded in
      London in 1781, and professedly based on the traditions of
      the ancient Druids. Lodges or groves of the society are
      established in other countries.
      [1913 Webster]

   Druid stones, a name given, in the south of England, to
      weatherworn, rough pillars of gray sandstone scattered
      over the chalk downs, but in other countries generally in
      the form of circles, or in detached pillars.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Druid
    n 1: a pre-Christian priest among the Celts of ancient Gaul and
         Britain and Ireland

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "Druid":
   Cassandra, Druidess, Parsi, Sabaist, Zoroastrian, animal worshiper,
   anthropolater, arborolater, archaeolater, astrologer, augur,
   calamity howler, chthonian, crystal gazer, daduchus,
   demon worshiper, demonolater, dendrolater, devil worshiper,
   divinator, diviner, divineress, epopt, fetishist, fire worshiper,
   flamen, forecaster, foreknower, foreseer, foreshower, foreteller,
   fortuneteller, geomancer, haruspex, hierodule, hierophant, hieros,
   idol worshiper, idolater, idolatress, idolatrizer, idolist,
   idolizer, mystes, nature worshiper, ophiolater, palmist,
   phallic worshiper, predictor, prefigurer, presager, prognosticator,
   prophesier, prophet, prophet of doom, prophetess, psychic,
   pyrolater, pythoness, religious prophets, seer, seeress,
   snake worshiper, soothsayer, star worshiper, sun worshiper,
   theriolater, therolater, tree worshiper, vates, weather prophet,
   zoolater