In the context of a down jacket's outer fabric, "denier" refers to a measurement of the thickness or weight of the yarn used in the fabric. Denier is a unit of measure for the linear mass density of fibers, and in the textile industry, it is commonly used to express the thickness of individual threads or filaments in fabrics.

A higher denier value typically indicates a thicker and more durable fabric. However, it's important to note that the choice of denier for a down jacket's outer fabric involves a balance between durability, weight, and intended use. Lightweight and packable jackets may use lower denier fabrics to reduce overall weight, while jackets designed for more rugged conditions may feature higher denier fabrics for increased durability.