由・因・縁|よしyoshi[1]atamadaka (head-high)

1. nounreason; significance; cause
2. nounpiece of information that one has heard; I hear that ...; it is said that ...

• Synonym理由りゆうNeutralThe everyday word for 'reason,' fine in any setting. Using 「由」 for a reason sounds archaic; today it mostly means 'I hear that,' as in 「お元気の由」.
• Synonym訳わけNeutralCommon in speech for a cause, reason or meaning, as in 「訳がわからない」. 「由」 is more literary and turns up mainly in letters.
• Synonym根拠こんきょNeutralThe grounds or evidence backing a judgment; also a base or stronghold. 「由」 never means grounds — it points to a reason or to hearsay.
• Synonym事由じゆうWrittenA written, officialese term — used in documents and law for the reason behind a matter, as on application forms. 「由」 belongs mainly to letter-writing.
• Synonym原因げんいんNeutralThe cause producing a result, typical with accidents and illness. 「由」 leans toward reasons and hearsay and is not used for the cause of an accident.
• Synonym所以ゆえんWrittenWritten prose for the grounds on which something is so, in set phrases like 「〜の所以である」. 「由」 has no such emphatic usage.
• Synonym原由げんゆWrittenA rare, literary word for a cause or origin; in everyday speech and writing 「原因」 or 「理由」 does the job instead.
• Related理屈りくつNeutralLogic or reasoning, often with a negative tinge of quibbling or strained arguments. 「由」 has no such pejorative sense; it covers reasons, hearsay or means.
• Related由来ゆらいNeutralA thing's origin or history; also an adverb meaning 'originally, by nature'. 「由」 covers reasons or news one has heard, not origins.
• Related道理どうりNeutralReason as what is right or natural, as in 「それも道理だ」. 「由」 points to a concrete reason or to something heard, not to what is fitting.
• Related筋合すじあいNeutralMostly in negatives: the reason or standing for why something ought to be so, as in 「言われる筋合いはない」. 「由」 has no such usage.
• Related裏付けうらづけNeutralEvidence or backing that supports a claim, as in 「裏付けを取る」. 「由」 covers reasons and hearsay but carries no idea of proof.
• RelatedわけあいNeutralUncommon: the reason and circumstances behind how something came about. In daily use just say 「訳」 or 「事情」.

For an everyday 'reason' say 「理由」; in casual speech, 「訳」; for what caused something, 「原因」; for grounds or evidence, 「根拠」 or 「裏付け」. 「由」 itself is archaic for 'reason' and today appears mostly in letters meaning 'I hear that,' as in 「お元気の由」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. （よし）（古語こご・雅語がご）理由りゆう。
2. （よし）（古語こご・雅語がご）手段しゅだん。
3. （よし）（古語こご・雅語がご・文書ぶんしょ語ご）伝聞でんぶんした内容ないよう。