The Collaborative International Dictionary of English v.0.48:

Systematic \Sys`tem*at"ic\, Systematical \Sys`tem*at"ic*al\, a.
   [Gr. ?: cf. F. syst['e]matique.]
   1. Of or pertaining to system; consisting in system;
      methodical; formed with regular connection and adaptation
      or subordination of parts to each other, and to the design
      of the whole; as, a systematic arrangement of plants or
      animals; a systematic course of study.
      [1913 Webster]

            Now we deal much in essays, and unreasonably despise
            systematical learning; whereas our fathers had a
            just value for regularity and systems. --I. Watts.
      [1913 Webster]

            A representation of phenomena, in order to answer
            the purposes of science, must be systematic.
                                                  --Whewell.
      [1913 Webster]

   2. Proceeding according to system, or regular method; as, a
      systematic writer; systematic benevolence.
      [1913 Webster]

   3. Pertaining to the system of the world; cosmical.
      [1913 Webster]

            These ends may be called cosmical, or systematical.
                                                  --Boyle.
      [1913 Webster]

   4. (Med.) Affecting successively the different parts of the
      system or set of nervous fibres; as, systematic
      degeneration.
      [1913 Webster]

   Systematic theology. See under Theology.
      [1913 Webster]

WordNet (r) 3.0 (2006):

systematic
    adj 1: characterized by order and planning; "the investigation
           was very systematic"; "a systematic administrator" [ant:
           unsystematic]
    2: of or relating to taxonomy; "taxonomic relations"; "a
       taxonomic designation" [syn: taxonomic, taxonomical,
       systematic]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "systematic":
   accordant, alike, analytical, arranged, automatic, balanced,
   businesslike, consistent, consonant, constant, continuous,
   correspondent, equable, equal, even, flat, formal, habitual,
   harmonious, homogeneous, immutable, in hand, invariable, level,
   logical, measured, mechanical, methodic, methodical, monolithic,
   normal, of a piece, ordered, orderly, organized, persistent,
   planned, regular, regular as clockwork, robotlike, routine, smooth,
   stable, standard, standardized, steadfast, steady, symmetrical,
   systematized, unbroken, unchangeable, unchanged, unchanging,
   undeviating, undifferentiated, undiversified, uniform, unruffled,
   unvaried, unvarying, usual, well-ordered, well-regulated