Therein (adverb)

1. In or into that particular place, situation, or thing.
2. In or into that which has been mentioned or indicated.

Origin:

Derived from the word "there" meaning "in that place" and "in" meaning "inside".

Examples:

1. The solution to the problem is therein.
2. The key to the success is therein.
3. The information you need is therein.
4. The secret to his success is therein.
5. The cause of the problem is therein.