Source : Webster's Revised Unabridged Dictionary (1913)

Hereby \Here*by"\, adv.
   1. By means of this.

            And hereby we do know that we know him. --1 John ii.
                                                  3.

   2. Close by; very near. [Obs.] --Shak.

Source : WordNet®

hereby
     adv : (formal) by means of this; "I hereby declare you man and
           wife" [syn: {herewith}]