Source : Webster's Revised Unabridged Dictionary (1913)

Therein \There*in"\, adv.
   In that or this place, time, or thing; in that particular or
   respect. --Wyclif.

         He pricketh through a fair forest, Therein is many a
         wild beast.                              --Chaucer.

         Bring forth abundantly in the earth, and multiply
         therein.                                 --Gen. ix. 7.

         Therein our letters do not well agree.   --Shak.

Source : WordNet®

therein
     adv : (formal) in or into that thing or place; "they can read
           therein what our plans are" [syn: {in this}, {in that}]