What is the name of the skin secretion that helps to keep it supple and waterproof?

The term that refers to the skin secretion responsible for keeping the skin supple and waterproof is sebum. Sebum is an oily substance produced by the sebaceous glands in the skin. It serves several important functions, including moisturizing the skin and creating a barrier that helps prevent water loss. This natural oil not only maintains skin elasticity but also plays a crucial role in protecting the skin from external factors such as dust and bacteria.

Other options do have relevant roles in skin health, but they do not primarily function as moisturizers or waterproofing agents. For example, keratin is a protein found in skin, hair, and nails that provides structural strength rather than moisture. Sweat, produced by sweat glands, primarily aids in thermoregulation through evaporative cooling and is not designed to waterproof the skin. While oil can contribute to skin moisture, it is more of a general term and does not specify the specific type of oil that serves this function. Thus, sebum is the most accurate choice when addressing the secretion that keeps the skin supple and waterproof.