Source : Webster's Revised Unabridged Dictionary (1913)

Thereby \There*by"\, adv.
   1. By that; by that means; in consequence of that.

            Acquaint now thyself with him, and be at peace;
            thereby good shall come unto thee.    --Job xxii.
                                                  21.

   2. Annexed to that. ``Thereby hangs a tale.'' --Shak.

   3. Thereabout; -- said of place, number, etc. --Chaucer.

Source : WordNet®

thereby
     adv : by that means or because of that; "He knocked over the red
           wine, thereby ruining the table cloth"