Plastic refers to a group of synthetic or semi-synthetic materials that have been engineered to achieve specific properties. The most notable characteristics of plastics are its low weight, machinability, corrosion resistance, and typically good thermal and electrical insulating properties. Certain plastic grades also offer optical transparency similar to glass, low-friction or self-lubricating surfaces, and exceptional impact resistance. Depending on their properties, certain plastic grades can be used as alternatives to metal, glass, and ceramic. Unlike metal, plastic may experience creep, which is deformation caused by longtime exposure to a constant load.