The Collaborative International Dictionary of English v.0.48:

Synthetic \Syn*thet"ic\, Synthetical \Syn*thet"ic*al\, a. [Gr.
   ?: cf. F. synth['e]tique.]
   1. Of or pertaining to synthesis; consisting in synthesis or
      composition; as, the synthetic method of reasoning, as
      opposed to analytical.
      [1913 Webster]

            Philosophers hasten too much from the analytic to
            the synthetic method; that is, they draw general
            conclusions from too small a number of particular
            observations and experiments.         --Bolingbroke.
      [1913 Webster]

   2. (Chem.) Artificial. Cf. Synthesis, 2.
      [1913 Webster]

   3. (Zool.) Comprising within itself structural or other
      characters which are usually found only in two or more
      diverse groups; -- said of species, genera, and higher
      groups. See the Note under Comprehensive, 3.
      [1913 Webster]

   Synthetic language, or Synthetical language, an
      inflectional language, or one characterized by grammatical
      endings; -- opposed to analytic language. --R. Morris.
      [1913 Webster]

WordNet (r) 3.0 (2006):

synthetical
    adj 1: involving or of the nature of synthesis (combining
           separate elements to form a coherent whole) as opposed to
           analysis; "limnology is essentially a synthetic science
           composed of elements...that extend well beyond the limits
           of biology"- P.S.Welch [syn: synthetic, synthetical]
           [ant: analytic, analytical]
    2: of a proposition whose truth value is determined by
       observation or facts; "`all men are arrogant' is a synthetic
       proposition" [syn: synthetic, synthetical] [ant:
       analytic, analytical]