The Collaborative International Dictionary of English v.0.48:

Zealous \Zeal"ous\ (?; 277), a. [LL. zelosus. See Zeal.]
   [1913 Webster]
   1. Filled with, or characterized by, zeal; warmly engaged, or
      ardent, in behalf of an object.
      [1913 Webster]

            He may be zealous in the salvation of souls. --Law.
      [1913 Webster]

   2. Filled with religious zeal. [Obs.] --Shak.
      [1913 Webster] -- Zeal"ous*ly, adv. -- Zeal"ous*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

zealous
    adj 1: marked by active interest and enthusiasm; "an avid sports
           fan" [syn: avid, zealous]