Source : Webster's Revised Unabridged Dictionary (1913)

Synclinal \Syn*cli"nal\, a. [Gr. ? to incline together; sy`n
   with + ? to incline.]
   1. Inclined downward from opposite directions, so as to meet
      in a common point or line.

   2. (Geol.) Formed by strata dipping toward a common line or
      plane; as, a synclinal trough or valley; a synclinal fold;
      -- opposed to anticlinal.

   Note: A downward flexure in the case of folded rocks makes a
         synclinal axis, and the alternating upward flexure an
         anticlinal axis.

Synclinal \Syn*cli"nal\, n. (Geol.)
   A synclinal fold.

Source : WordNet®

synclinal
     adj : sloping downward toward each other to create a trough [ant:
           {anticlinal}]