It is a type of material that stretches and is used especially for making clothes fit tightly.
American Apparel 8308W - Women's Cotton Spandex Tank
Badger 4280 - Poly/Spandex 1/4 Zip Lightweight Pullover
Flexfit 6477 - Wooly Blend 6-Panel Cap
Weatherproof 6500 - Soft Shell Jacket
Flexfit 6577CD - Cool & Dry® Piqué Mesh Cap
Flexfit 5511UP - Unipanel™ Trucker Cap
Next Level 3533 - Ladies' The Jersey Tank
Flexfit 180 - Delta Seamless Cap