The answer is synthetic fiber. Milady names the specific material — Kanekalon, a synthetic fiber — that ready-to-wear wigs are usually made from.
Wool, angora, and yak hair are all real materials named elsewhere in the same chapter, but they show up in the context of theatrical or fashion wigs, where human hair is often blended with animal or synthetic hair. Ready-to-wear wigs, built for everyday convenience rather than custom styling, are a synthetic-fiber product by default.
Milady Standard Barbering, 6th ed., Ch. 15 (Men's Hair Replacement), p. 505: "Ready-to-wear wigs are usually made of the synthetic fiber Kanekalon."
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