annulus

This term is primarily used in technical, scientific, and medical contexts to describe a precise geometric shape. It carries a clinical or mathematical tone, distinguishing it from common words like ring or circle by emphasizing the area between two concentric boundaries rather than just a simple line.

In anatomy, the word specifically identifies structural bands of tissue, such as those found in the spine. Because it is a specialized Latinate term, it is rarely encountered in casual conversation and is almost exclusively reserved for academic papers, surgical reports, or astronomical observations.