Collar which lies only on the shoulders, and does not rise onto the neck.
Zamkoff, Price, 1990
Context:
Flat collars have neckline curves that are the same shape as the neckline of the garment. This allows them to lie flat on the garment with no roll or stand onto neck. Flat collars are popular on children’s clothes because children have short necks. On adult garments they create a youthful feel.