annulus

Latin annulus meant a ring, especially a finger ring, in texts from the late Roman Republic and early Empire. It is a diminutive form built on anus, an older Latin word for a ring or circular band. Roman writers used annulus for jewelry, seals, and any small ring-shaped object. The form was stable by the 1st century BCE and plainly belonged to ordinary educated Latin.

As Latin moved into technical writing, annulus kept its literal shape while widening its use. Anatomists used it for ring-like structures in the body, and mathematicians later used it for the region between two concentric circles. Medieval and early modern Latin preserved the word because it was exact and compact. That scientific habit carried it forward long after everyday spoken Latin disappeared.

English took annulus from Latin in learned writing rather than from street speech. It appears in early modern scientific English for anatomy and geometry, where Latin terms still set the standard. By the 17th century, English writers were using annulus for ring-shaped parts and forms that needed a precise label. The word stayed narrow in scope, but it stayed useful.

Today annulus is still the technical name for a ring or ring-shaped zone. In anatomy it names structures such as the annulus fibrosus, and in mathematics it names the area between nested circles. Its history is unusually direct: Latin named the shape, and modern science kept the name. The word has changed little because the shape has not.