the formation of rings
a ringlike formation or part
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
How to use annulation in a sentence
There is very little appearance of annulation of the stem, and no evidence of the "stem increasing in diameter distally."
The absence of setae and the great secondary annulation render the mapping of the segments a subject of some difficulty.
The distinguishing feature is the transverse annulation of the bell.Marine Protozoa from Woods Hole | Gary N. Calkins