/ðeɚˈın/ adv
formal
1 : in or into that place or thing
The insurance covers the apartment and all the property therein.
His name was mentioned in the article, although he was incorrectly described therein as a medical doctor.
2 : in that statement, fact, or detail
They have to make a decision soon. Therein lies the problem. [=that is why there is a problem] If they act too quickly, they run the risk of choosing the wrong plan.
Useful english dictionary. 2012.