Zylon is a trademarked name for a range of thermoset polyurethane materials manufactured by the Zylon corporation. These are members of the synthetic polymer family of materials.
The particular subset that Zylon is derived from are the EBXL-TPU plastics. (From Electron Beam cross-linked ThermoPlastic Polyurethane).
Vaguely related to Kevlar and nylon, Zylon is used in a number of applications that require very high strength with excellent thermal stability. Tennis raquets, various medical applations, and some of the martian rovers are some of the more well known instances.
Zylon is also used in body armor. Zylon-based body armor has recently become controversial, as some studies report that it may degrade rapidly, leaving wearers with significantly less protection than expected. American Body Armor Holdings Inc. has recalled its existing Zylon-based products, and has decreased the rated lifespan of new vests from 60 months to 30 months.