is a type of thermoplastic that is shaped with heat. Thermoplastic has been around for a long time, but unlike other forms of thermoplastic you don't need to put this in your oven, and it doesn't smell like burning DEATH when you work with it. (It's non-toxic too, which cosplayers tend to ignore for their hobby, but is a nice side bonus anyway). It is also unique because, on the bias (diagonal line) it will stretch when heated, allowing full, round shapes and bell curves.
It has a mesh grid inside the plastic which allows it to stretch on the bias unlike other plastics. One side of the plastic shows the grid (a texture similar to duct tape) while the other side is smooth plastic with an eggshell-like texture.