The Collaborative International Dictionary of English v.0.48:

Ideology \I`de*ol"o*gy\, n. [Ideo- + -logy: cf. F.
   id['e]ologie.]
   1. The science of ideas. --Stewart.
      [1913 Webster]

   2. (Metaph.) A theory of the origin of ideas which derives
      them exclusively from sensation.
      [1913 Webster]

   Note: By a double blunder in philosophy and Greek,
         id['e]ologie . . . has in France become the name
         peculiarly distinctive of that philosophy of mind which
         exclusively derives our knowledge from sensation. --Sir
         W. Hamilton.

   3. A set or system of theories and beliefs held by an
      individual or group, especially about sociopolitical goals
      and methods to attain them; in common usage, ideology is
      such a set of beliefs so strongly held by their adherents
      as to cause them to ignore evidence against such beliefs,
      and thus fall into error -- in this sense it is viewed as
      a negative trait; contrasted to pragmatism, and distinct
      from idealism.
      [PJC]

WordNet (r) 3.0 (2006):

ideology
    n 1: an orientation that characterizes the thinking of a group
         or nation [syn: political orientation, ideology,
         political theory]
    2: imaginary or visionary theorization