The term 'layer of skin' refers to the three distinct layers that make up the human skin. The EPIDERMIS, which is the outermost layer, is composed of nine letters and is responsible for providing a protective barrier against the environment. The DERMIS, which is the middle layer, is composed of six letters and contains sweat glands, hair follicles, and blood vessels. Lastly, the DERMA, which is the innermost layer, is composed of five letters and is responsible for providing nourishment to the skin. Together, these three layers form the human skin and are essential for its proper functioning.