What is annulus?

Answer 1

We all have cartilaginous discs that separate the spinal bones, or vertebraes, from each other. These discs allow for movement within the spine. Each disc has a fibrous outside, called the annulus fibrosus, and a liquid center, called the nucleus pulposus. Think of a disc as a jelly donut. The jelly is the nucleus and the donut is the annulus. If the fibers of the inner annulus tear, the jelly nucleus, which is tightly held in the center, follows the path of least resistance and pushes towards the wall of the annulus. When this happens, the wall of the annulus is warped and appears to bulge or protrude. This is known as a protruding annulus or ,in other words, a bulging disc. If the annulus continues to weaken and tear, the nucleus has potential to eject from the disc. The annulus will no longer be bulging, as the wall has opened and the jelly nucleus has squeezed out or herniated. This now constitutes a herniated disc.

Answer 2

In Latin it means "little ring".