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1. a numerical quantity that is not a whole number (e.g. 1/2, 0.5).

Mathematics〘数〙 分数; (数の)比

2. a small or tiny part, amount, or proportion of something:

(全体に対して)部分, 小片, 断片; ほんの一部, わずか; ; 〖a ～; 副詞的に〗 ちょっと, ほんの少し

e.g. he hesitated for a fraction of a second

e.g. her eyes widened a fraction.

a dissenting group within a larger one.

3. Chemistry each of the portions into which a mixture may be separated by a process in which the individual components behave differently according to their physical properties.

〘化〙 留分

4. (usually the Fraction) (in the Christian Church) the breaking of the Eucharistic bread.

〖通例the F-〗〘キリスト教〙 (聖餐（せいさん）式などで)パンを分割すること

late Middle English: via Old French from ecclesiastical Latin fractio(n-) ‘breaking (bread)’, from Latin frangere ‘to break’.