What part of speech is “atoll”

an atoll is a circular or horseshoe-shaped island, consisting of a coral reef surrounding a central lagoon. The word is derived from a Maldivian word meaning 'island'. An atoll may be part of an archipelago as well, which is a group of smaller islands. Atolls are typically found in the tropical climates and oceans.

atolls exist mostly in tropical parts of the world, typically near the equator. When referring to an atoll, there is usually a word or two going before it to refer to the type of atoll that it is. For example, one may refer to a coral atoll, or fringing atoll. When describing the atoll, the words 'circular,' 'horseshoe-shaped,' and 'central lagoon' are used.

On our vacation, we visited a Caribbean coral atoll that was beautiful and peaceful.

When my family moved, I remembered fondly the many hours we spent at our favorite fringing atolls.

We decided to rent kayaks and take a tour around the horseshoe-shaped atoll.

when talking about atolls in a general sense, do not refer to them as 'islands,' as this is an inaccurate description and implies that the landmass is not a lagoon surrounded by a coral reef as is a typical atoll.