The Collaborative International Dictionary of English v.0.48:

Hereby \Here*by"\, adv.
   1. By means of this.
      [1913 Webster]

            And hereby we do know that we know him. --1 John ii.
                                                  3.
      [1913 Webster]

   2. Close by; very near. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hereby
    adv 1: (formal) by means of this; "I hereby declare you man and
           wife" [syn: hereby, herewith]