The Collaborative International Dictionary of English v.0.48:
Herein \Here*in"\, adv. [AS. h?rinne.] In this. [1913 Webster] Herein is my Father glorified, that ye bear much fruit. --John xv. 8. [1913 Webster]WordNet (r) 3.0 (2006):
herein adv 1: in this place or thing or document; "I shall discuss the question herein"