therein
adv.in that place, document, or situation. You use this formal word to point back to something you just mentioned.
adv.in or into that place, thing, or circumstance. Often used in legal or academic contexts to refer back to a previously mentioned statement or document.
“The contract is clear, and therein lies the problem.”
“The report identifies several safety risks and the specific costs associated therein.”
“The philosopher argued that the soul is distinct from the body and that therein resides the capacity for true moral agency, independent of physical impulses.”
Formal or literary register; typically follows the noun or clause it refers to.
“And therein lies the muscle that every entrepreneur must grow.”
From Middle English theryn, ther-inne, þer-in, from Old English þǣrin, þǣrinne, from Proto-West Germanic þārin, þārinna, from Proto-Germanic þarin, þarinna, equivalent to there + in. Cognate with Saterland Frisian deerin (“therein”), West Frisian dêryn (“therein”), Dutch daarin (“therein”), German Low German daarin (“therein”), German darin, drin (“therein”), Danish deri (“therein”), Swedish däri (“therein”), Icelandic þar í (“therein”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.