The long, lean muscle inside a cow's loin, the most tender cut, with no fat or sinew. It is stir-fried quickly, served sizzling on an iron plate, or cooked as fillet steak.
Beef tenderloin tastes good only when it's tender.
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柳 (willow) lent its name to the cut because tenderloin strips are long, slender and pliable like willow leaves. Same logic for 鸡柳 (chicken tenders) and 鱼柳 (fish fingers / fish strips).