Therein means inside that thing or document. Therein lies the problem, described therein, the rights contained therein.
It belongs to legal drafting, where it points back to a document already named without repeating it. Therein lies the problem is the one phrase everybody knows, and it is deliberately old-fashioned when anybody uses it in speech.
1adverbthereinInside that thing or document.
“Please read the attached contract and the conditions contained therein.”
therein is a reference.
Old English thaerin, there plus in. Therefore, thereby and thereof are made the same way.
Proto-Germanic
*þarin
Proto-West Germanic
*þārin
Old English
þǣrin
Middle English
theryn
English, today
therein
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 stq 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 from Wiktionary, CC BY-SA