Single layer is a term used to describe a type of construction or material that consists of only one layer. In the context of a crossword puzzle, the terms HEN (3 letters) and PLY (3 letters) are solutions for the term single layer. HEN is an abbreviation for henry, which is a unit of electrical inductance, while PLY is an abbreviation for plywood, which is a type of wood panel made from thin sheets of wood veneer.