Meaning of “therein” - Learner’s Dictionary

therein

adverb uk us /ˌðeəˈrɪn/ formal

in a particular document or place:

We recommend that you study the report and the proposals contained therein.
therein lies sth

because of the reason that has just been given:

But the medicines are expensive, and therein lies the problem.

(Definition of “therein” from the Cambridge Learner’s Dictionary © Cambridge University Press)