annulus fibrosus

noun

annulus fi·​bro·​sus -fī-ˈbrō-səs, -fi- 

variants also anulus fibrosus

ˈan-yə-ləs- 

: a ring of fibrous tissue (such as that surrounding a heart valve)

especially : the outer portion of an intervertebral disc that surrounds the nucleus pulposus and is composed chiefly of layers of collagen fibers

Disks between the vertebrae have a soft center—a nucleus pulposus—and a tougher exterior, annulus fibrosus.

—Matthew Sturdevant, The Hartford (Connecticut) Courant