The Collaborative International Dictionary of English v.0.48:

Thematic \The*mat"ic\, a. [Gr. ?: cf. F. th['e]matique.]
   1. (Gram.) Of or pertaining to the theme of a word. See
      Theme, n., 4.
      [1913 Webster]

   2. (Mus.) Of or pertaining to a theme, or subject.
      [1913 Webster]

   Thematic catalogue (Mus.), a catalogue of musical works
      which, besides the title and other particulars, gives in
      notes the theme, or first few measures, of the whole work
      or of its several movements.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thematic
    adj 1: relating to or constituting a topic of discourse
    2: of or relating to a melodic subject [ant: unthematic]