An atoll unit is a geographical term used to describe a small island or group of islands. An atoll unit can be referred to as an island (6 letters), ISLE (4 letters) or ISLET (5 letters). An island is the largest of the three, and is typically surrounded by water on all sides. An ISLE is smaller than an island and is also surrounded by water, while an ISLET is the smallest of the three and is usually just a small rocky area surrounded by water.