FR4 is a designation for a printed circuit board (PCB) substrate material widely used in the electronics industry. Specifically:

1.  FR stands for "Flame Retardant." This indicates that the material meets specific flame retardant safety standards (usually UL94 V-0 rating), is self-extinguishing, and can prevent the spread of fire to some extent.

2.  4 is a specific formulation code under this flame retardant rating, referring to "epoxy resin impregnated woven fiberglass cloth."

Therefore, FR4 is essentially a flame-retardant composite substrate using epoxy resin as a binder and fiberglass cloth as reinforcement. Due to its excellent mechanical strength, electrical insulation properties, and flame-retardant characteristics, it has become one of the most commonly used materials in the manufacture of ordinary PCBs.

It is also used in plastic molds, injection molds, machinery manufacturing, molding machines, drilling machines, injection molding machines, motors, ICT fixtures, and table grinding pads.