Source : Webster's Revised Unabridged Dictionary (1913)

Herein \Here*in"\, adv. [AS. h?rinne.]
   In this.

         Herein is my Father glorified, that ye bear much fruit.
                                                  --John xv. 8.

Source : WordNet®

herein
     adv : in this place or thing or document; "I shall discuss the
           question herein"