The term 'synthetic used to insulate wet suits' is referring to neoprene, an 8-letter word. Neoprene is a synthetic rubber material that is commonly used in the production of wet suits and other forms of protective clothing. It is known for its excellent insulation properties, making it an ideal choice for keeping the wearer warm and dry in cold water. Neoprene is also resistant to chemicals, oils, and UV radiation, making it a durable and long-lasting material.