An atoll was a type of island. Notable Earth atolls included Bikini Atoll. (ENT: "Proving Ground")
In 1937, Amelia Earhart and Fred Noonan's airplane was running out of gasoline, causing them to look for an atoll to land on just prior to being abducted by aliens. She related this to the crew of USS Voyager in 2371. (VOY: "The 37's")