Ply (layer)

A ply is a layer of material which has been combined with other layers in order to provide strength. The number of layers is indicated by prefixing a number, for example 4-ply, indicating material composed of 4 layers.
Etymology.
The word "ply" derives from the French verb "plier", "to fold", from the Latin verb "plico", from the ancient Greek verb "πλέκω".