The term 'strong waterproof elastic material' can be summarized as RUBBER, which is a 6-letter word. RUBBER is a versatile material that has been used for centuries and is made from the sap of certain trees. It is known for its strength, waterproofing, and elasticity, making it ideal for a variety of applications such as tires, seals, and gaskets. RUBBER is also used in many consumer products, including shoes, clothing, and toys.