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Definition of annulus:
Synonyms:
closed chain, ring, aura, bird, anulus, glory, nimbus, doll, tintinnabulation, gang, wench, chick, hoop, dame, aureole, pack, mob, sinker, skirt, anchor ring, ringing, donut, doughnut, band, gloriole, halo.

roundabout (part of speech: noun)
 halo,
 hemisphere,
 areola,
 circuit,
 circus,
 circle,
 compass,
 circumference,
 circuitousness,
 orbit,
 circularity,
 ring,
 roundabout,
 loop
Usage examples:

In Diagram 2 I have introduced a square as it often occurs in ancient drawings of the Monad; which was my reason for believing that the symbol had mathematical meanings, since it will be found to demonstrate the fact that the area of the outer ring or annulus is exactly equal to the area of the inner circle.
 "Amusements in Mathematics", Henry Ernest Dudeney. 
9, marks about the maximum spread of the liquid, which, subsiding in the middle, afterwards flows into an annulus or rim with a very thin central film, so thin, in fact, as often to tear more or less irregularly.
 "The Splash of a Drop", A. M. Worthington. 
Now imagine the cylinder bent into an annulus.
 "The Splash of a Drop", A. M. Worthington.