noun
Refers to the reason, origin, or background behind something, often implying a historical or logical connection. Can also mean a relationship or affinity between people or things. Slightly formal/literary.
この町の名前の由縁を調べている。
I'm researching the origin of this town's name.
彼が「鉄人」と呼ばれる由縁は、その驚異
The reason he's called 'Iron Man' lies in his incredible stamina.
理由 is the common word for 'reason' in everyday contexts. 由縁 is more formal and often implies a deeper historical or etymological background.
由来 also means 'origin' or 'derivation', but is more commonly used in compound phrases like ～に由来する. 由縁 can carry a nuance of 'connection' or 'affinity' that 由来 lacks.
Composed of 由 (reason, cause) and 縁 (connection, edge). The combination emphasizes a causal or relational link. The exact historical derivation is uncertain, but the kanji are conventionally associated with the meaning.