Windstopper is a windproof breathable fabric laminate made by W. L. Gore & Associates. One of its most common applications is a lamination with Polar fleece, to compensate for fleece's lack of wind resistance.
Unlike Gore's well-known Gore-Tex laminate, Windstopper is not waterproof.
Many Windstopper garments are marketed as "softshells" suited to high-output aerobic activities such as running, cycling or cross-country skiing. Because they are not waterproof, they are more breathable than traditional Gore-Tex "hardshell" clothing. However, because they are based on a solid laminate layer, they remain inherently less breathable than other wind-resistant "softshell" fabrics made by Polartec or Schoeller, which rely on built-in properties of the fabric and weave rather than laminates.
Like most softshell fabrics, Windstopper products are typically coated with DWR (Durable Water Repellent), which provides them with a modest degree of water resistance. They will wet through in heavy rainfall but can be worn comfortably in light drizzle.