A plastic which is available in several forms: Toughened polystyrene is a rigid plastic in sheet or board form. Foam core or foam board is a strong, stiff, resilient, and lightweight board of polystyrene laminated with paper on both of its sides. Expanded polystyrene is a lightweight, granular mass, usually worked in sheets or blocks, but also available in loose granules.(pr. PAH-lee-STI:-reen)