The term EXTERNAL LAYER can be broken down into two crossword solutions: SKIN (4 letters) and VENEER (6 letters). SKIN is the outermost layer of the human body, which serves as a protective barrier against the environment. VENEER is a thin layer of material applied to the surface of an object to give it a more attractive appearance. Both SKIN and VENEER are examples of external layers that serve a protective or aesthetic purpose.