atoll

名词 atoll:

1. an island consisting of a circular coral reef surrounding a lagoon

a.toll

n.（名词）
A ringlike coral island and reef that nearly or entirely encloses a lagoon.
环礁：一种环形的珊瑚岛或珊瑚链，接近或完全围绕于浅泻湖

来源：
Perhaps ultimately from Tamil aúar [to be close together, thick, crowded]
可能最终源自 泰米尔语 aúar [围在一处的，厚实的，拥挤的]