In the hierarchy of coastal geography, an islet is the most common term for a landmass smaller than an isle. While an "isle" is a small island, an "islet" is typically too small to support permanent human habitation and often lacks significant vegetation. Even smaller than an islet is a skerry, which is a small, rocky island or reef that may be partially submerged during high tide. In some regions, a cay (or key) is used to describe a low-elevation, small island formed on the surface of a coral reef, though these can vary in size. Furthermore, an ait (or eyot) refers specifically to a small island found within a river or lake. In terms of literal size and functionality, a "skerry" or an "unnamed rock" represents the smallest classification of permanent land above sea level. These tiny landmasses are often critical for maritime navigation and as nesting grounds for seabirds, even if they are only a few square meters in total area and consist entirely of bare stone.