Airplane windows are made of polymethylmethacrylate (PMMA), commonly known as acrylic glass. This material is chosen for its exceptional strength, transparency, and lightweight properties. It is highly resistant to impact and cracking, which is crucial for withstanding the extreme pressure differences and temperatures experienced during flight. Unlike polyethene, PVC, or polypropylene, which are softer or less transparent, PMMA provides the necessary safety and optical clarity required for aircraft windows.

Final Answer: Polymethylmethacrylate