Source : Webster's Revised Unabridged Dictionary (1913)

Syntactic \Syn*tac"tic\, Syntactical \Syn*tac"tic*al\, a. [Cf.
   G. ? putting together. See {Syntax}.]
   Of or pertaining to syntax; according to the rules of syntax,
   or construction. -- {Syn*tac"tic*al*ly}, adv.

Source : WordNet®

syntactically
     adv : with respect to syntax; "syntactically ill-formed"