Ply
What is Ply in skateboarding?
A ply is each of the thin wood veneers that get glued and pressed together to form a deck. The standard is 7 plies of maple.
Extended definition
The veneers are glued with the grain alternating direction, and that cross-grain is where a deck's strength comes from. 8-ply decks are stiffer and get recommended to heavier riders; almost nobody goes past 9 because the board gets too heavy. Constructions like R7 swap the water-based glue for epoxy to gain lightness and stiffness.