To help explain the two steps of this operation, consider the vertebrae of the spine. Each one has a round hole in the middle for the spinal cord to pass through. The bumps you feel down your back are part of the vertebrae and are called the spinous processes. The part of the backbone on either side of the spinous process is the lamina. The lamina helps protect the spinal cord.
Between the vertebral bones is a spongy cushion called the disc. The disc acts like a shock absorber for your back. Sometimes it becomes injured and pushes against the spinal nerves inside the spinal canal. This can lead to pain in your back or down your leg. It can also cause nerve symptoms such as numbness or tingling in your leg or foot. Removing the disc is called a discectomy. ("Ectomy" means to remove).
To take pressure off the spinal cord or spinal nerves inside the spinal column, doctors may remove a portion of the vertebral bone (the lamina). This part of the operation is called a laminectomy. If the surgeon only removes the lamina on one side of the spinous process, the operation is called a hemilaminectomy. ("Hemi" means half.)