Merriam-Webster Collegiate Dictionary - annulus

 noun  (plural annuli; also -luses)  Etymology: Medieval Latin, from Latin anulus finger ring, from anus ring — more at anus  Date: 1563  1. ring  2. a part, structure, or marking resembling a ring: as  a. a line of cells around a fern sporangium that ruptures the sporangium by contracting  b. a growth ring (as on the scale of a fish) that is used in estimating age