1.
Therein means contained in the place that has been mentioned. (FORMAL)
By burning tree branches, pine needles, and pine cones, many not only warm their houses but improve the smell therein.
ADV: n ADV
2.
Therein means relating to something that has just been mentioned. (FORMAL)
Afternoon groups relate to the specific addictions and problems therein.
ADV: n ADV
3.
When you say therein lies a situation or problem, you mean that an existing situation has caused that situation or problem. (FORMAL or OLD-FASHIONED)
Santa Maria di Castellabate is barely mentioned in guidebooks; therein lies its charm.
PHRASE: V inflects, PHR n