Side Play is the very slight shifting of leaves in the direction perpendicular to the pin.
The curl of the leaves must not grip the pin too tightly, or the hinge will not rotate.
This small allowance for rotation also allows a tiny amount of side play.
Side play is usually only noticed when attempting to hold tight tolerance on dimensions
that cross the pin. For example, the distance between a hole on one leaf to a hole
on the other leaf is going to be subject to side play.