Noun. 1. polypropylene-a polymer of propylene used as a thermoplastic molding material. polypropene. plastic-generic name for certain synthetic or semisynthetic materials that can be molded or extruded into objects or films or filaments or used for making e.g. coatings and adhesives.oun. 1. polypropylene-a polymer of propylene used as a thermoplastic molding material. polypropene. plastic-generic name for certain synthetic or semisynthetic materials that can be molded or extruded into objects or films or filaments or used for making e.g. coatings and adhesives.