Ply: A measure of the number of individual yarns twisted together to produce the finished carpet yarn. For example, a two-ply yarn means that each tuft consists of two yarns twisted together. For cut-pile carpets plied yarns must be heatset to prevent untwisting under foot traffic.
or
A term to denote the number of thicknesses or layers of roofing felt, veneer in plywood, or layers in built-up materials, in any finished piece of such material.
Basically ply just means layer / thickness.
Take your pick here:
PLY DEFINITIONS