Meaning

noun a toroidal shape

noun (Fungi) a remnant of the partial veil that in mature mushrooms surrounds the stem like a collar

originLearned borrowing from Medieval Latin ānnulus, a misspelling of Latin ānulus (“ring, especially one worn on a finger”), from ānus (“ring”) (possibly from Proto-Indo-European *h₁eh₂-n-o- (“ring”), from an uncertain root) + -ulus (diminutive suffix). The plural form annuli is a learned borrowing from Medieval Latin ānnulī.

Etymology from Wiktionary, CC BY-SA 3.0