An atoll is a coral island

Coral island

A coral island is the result of an atoll whose lagoon has dried up or been filled in with coral sand and detritus. This state is typically the last in the life cycle of an island, the first being volcanic and the second being an atoll. Most of the world's coral islands are in the Pacific Ocean...

 (or islands) that encircles a lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

 partially or completely.
The word atoll comes from the Dhivehi

Dhivehi language

Maldivian is an Indo-Aryan language predominantly spoken by about 350,000 people in the Maldives where it is the national language. It is also the first language of nearly 10,000 people in the island of Minicoy in the Union territory of Lakshadweep, India where the Mahl dialect of the Maldivian...

 (an Indo-Aryan language spoken on the Maldive Islands

Maldives

The Maldives , , officially Republic of Maldives , also referred to as the Maldive Islands, is an island nation in the Indian Ocean formed by a double chain of twenty-six atolls oriented north-south off India's Lakshadweep islands, between Minicoy Island and...

) word atholhu (Dhivehi

Dhivehi language

Maldivian is an Indo-Aryan language predominantly spoken by about 350,000 people in the Maldives where it is the national language. It is also the first language of nearly 10,000 people in the island of Minicoy in the Union territory of Lakshadweep, India where the Mahl dialect of the Maldivian...

: އަތޮޅު, ˈət̪ɔɭu)^OED. Its first recorded use in English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 was in 1625 as atollon - Charles Darwin

Charles Darwin

Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs

The Structure and Distribution of Coral Reefs

The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs...

, the definition of atolls as "..circular groups of coral islets [...] and [the word] is synonymous with 'lagoon-island'." (1842, p. 2).