How to say "fiber" in Japanese

• ファイバーCommon
  nounThe katakana loanword for fiber as a material. Typical contexts are technical fields and product labeling — optical fiber, carbon fiber, supplements — where the borrowed term sounds natural.
• 繊維せんいJLPT N1approx.Common
  nounThe general word for fiber as a substance: the threadlike material of textiles, food, and body tissue. For dietary fiber, natural or synthetic fibers, or fiber discussed scientifically, this is the word to use.
  マイクロファイバークロスは、髪の毛かみのけの１００分ぶんの１という細ほそさの繊維せんいで編あみあげられたクロスです。↗

  Microfibre cloth is cloth woven from fibres as narrow as a hundredth of the width of a hair.

繊維 is the general word for fiber as a material — in textiles, food, or the body — while 「ファイバー」, the katakana loanword, belongs chiefly to technical and commercial contexts where the English term is used as is.

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

• ファイバーfiber
• 繊維せんいfibre
• 筋すじfiber