The Collaborative International Dictionary of English v.0.48:

Ruminative \Ru"mi*na*tive\, a.
   Inclined to, or engaged in, rumination or meditation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ruminative
    adj 1: deeply or seriously thoughtful; "Byron lives on not only
           in his poetry, but also in his creation of the 'Byronic
           hero' - the persona of a brooding melancholy young man";
           [syn: brooding, broody, contemplative,
           meditative, musing, pensive, pondering,
           reflective, ruminative]