Definition of the term Ply 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. Related Posts Definition of the term Relative Heat Gain The amount of heat gain through a glass product taking… Definition of the term Rise In stairs, the vertical height of a step or flight… Definition of the term RPM Revolutions per Minute.