Definition of annulus
1
: ring
2
: a part, structure, or marking resembling a ring: such 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
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Merriam-Webster unabridged
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Time Traveler for annulus
The first known use of annulus was in 1563
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