The Collaborative International Dictionary of English v.0.48:

Introspective \In`tro*spec"tive\, a. [Cf. F. introspectif.]
   [1913 Webster]
   1. Inspecting within; seeing inwardly; capable of, or
      exercising, inspection; self-conscious.
      [1913 Webster]

   2. Involving the act or results of conscious knowledge of
      physical phenomena; -- contrasted with associational.
      --J. S. Mill.
      [1913 Webster]

WordNet (r) 3.0 (2006):

introspective
    adj 1: given to examining own sensory and perceptual experiences
           [syn: introspective, introverted, self-examining]
           [ant: extrospective, extroverted]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "introspective":
   absorbed, abstracted, buried in thought, cogitative, cognitive,
   concentrating, concentrative, conceptive, conceptual,
   conceptualized, contemplating, contemplative, deliberating,
   deliberative, engaged in thought, engrossed, engrossed in thought,
   excogitating, ideative, immersed in thought, lost in thought,
   meditating, meditative, mental, museful, musing, noetic, occupied,
   pensive, pondering, prehensive, preoccupied, rapt, reflecting,
   reflective, ruminant, ruminating, ruminative, serious, sober,
   speculative, thinking, thought, thoughtful, wistful,
   wrapped in thought