同一|どういつdouitsu[0]heiban (flat)

1. no-adjective, na-adjectiveidentical; same; one and the same; equal
   これは私が先日せんじつなくしたのと同一どういつの鉛筆えんぴつである。↗

   This is the same pencil that I lost the other day.
2. no-adjective, na-adjectivefair; equal treatment; without discrimination

• Synonym同じおなじNeutralThe most common word for 'same', as in 「同じ本」; 「同一」 is more written-style and stresses being exactly one and the same.
• Synonym等しいひとしいNeutralFor equality in numbers or degree, as in 「長さが等しい」; centers on quantities and properties rather than identity.
• SynonymイコールNeutralLoanword for the equals sign or 'equals', as in 「AイコールB」; in casual speech it means equating two things.
• Synonym同然どうぜんNeutralFor being virtually the same, as in 「ただ同然」 or 「家族同然」; stresses practical equivalence, not exact identity.
• Synonym同質どうしつNeutralFor the same quality or nature, as in 「同質の材料」; centers on shared makeup.
• Related同様どうようNeutralMeans roughly the same or likewise, as in 「前回と同様に」; allows slight differences, where 「同一」 demands exact identity.
• Related同等どうとうNeutralFor the same degree or rank, as in 「同等の権利」; centers on equal standing or value, not identity.
• Related同格どうかくNeutralFor equal qualifications or status, and in grammar for apposition; the focus is on rank.
• Related等価とうかNeutralFor equal value, as in 「等価交換」; leans toward economics and logic.
• Related均質きんしつNeutralFor parts that are uniform in quality, as in 「均質な製品」; centers on internal evenness.
• Related一律いちりつNeutralFor treating everything by one standard, as in 「一律千円」; centers on uniform handling.
• Related均一きんいつNeutralFor quantities or degrees with no variation, as in 「均一価格」 or 「百円均一」.
• Related同値どうちWrittenWritten, technical term for equivalence in logic and mathematics.
• Related等値とうちWrittenWritten word for equal value, used mostly in mathematics and economics.
• Related同じようおなじようNeutralMeans similar or nearly the same, as in 「同じような服」; a looser notion than 「同一」.
• Related類同るいどうWrittenWritten word for being of the same kind and alike; rare in modern usage.
• RelatedクリソツCasualSlang, an inversion of 「そっくり」, for looking exactly alike; only in casual settings.
• Related同じいおなじいWrittenAn old-fashioned adjectival form of 「同じ」; almost never used today.
• Related斉一せいいつWrittenWritten word for uniformity, everything being alike; rare in modern usage.
• Related同質的どうしつてきWrittenFor sharing the same nature, as in 「同質的な集団」; common in sociological writing.
• Antonym別個べっこNeutralFor things that are separate and distinct, as in 「別個の問題」; the opposite of 「同一」.
• Antonym相違そういNeutralFor being different or a discrepancy, as in 「意見の相違」; the opposite of 「同一」.
• Antonym異同いどうWrittenWritten word for differences, or for whether things are the same or not, as in 「本文の異同」; the antonym of 「同一」 in the same–different contrast.

Use 「同一」 for being exactly one and the same or fully identical; in everyday speech 「同じ」 is the default. For roughly the same pick 「同様」, for numerical or degree equality 「等しい」, for equal status 「同等」, and for a uniform standard 「一律」. The opposite is 「相違」.

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

1. 二ふたつ以上いじょうの形状けいじょう、性質せいしつ等とうが同おなじであること。
2. 物事ものごとの程度ていどが差さがなく等ひとしいこと。