FR4 is a glass fiber epoxy laminate. It is the most commonly used PCB material. 1.60mm FR4 uses 8 layers of glassfiber material. It is usually transparent. The green color comes from the solder mask in the PCB finished product.
FR-4
Meaning of FR-4 in English
Electronics Terminology English vocabulary. Английский словарь терминологий электроники. 2012