Core
Also known as: Wheel core
What is Core in skateboarding?
The core is the rigid hub some wheels have at their centre, the spot where the bearings seat inside the urethane.
Extended definition
A cored wheel seats its bearings better and handles the pressure of the axle nut better. On soft or coreless wheels spacers are recommended, because the urethane gives when you tighten the nut. There are also dual-durometer wheels, with a harder core than the outside for speed plus grip.