Formal or literary word for the reason or cause behind something. Often used in the pattern 〜所以である or 〜所以だ.
The reason he succeeded lies in the fact that he spared no effort.
The reason this town is popular is its beautiful nature and quiet environment.
Standard kanji form for this word.
Kana spelling is common in everyday writing, especially when the kanji form is considered difficult or formal.
The kanji 所 (place) and 以 (by means of) combine to convey 'that by which something is so', i.e., the reason or grounds. The reading ゆえん is a kun'yomi-based compound.