A limpet is an aquatic snail with a shell that is broadly conical in shape. "Limpet" informally refers to any gastropod whose shell has no obvious coiling, like the coiling which can be seen in the shells of garden snails or winkles.

Although all limpets are members of the class Gastropoda, limpets are highly polyphyletic, meaning that the various groups that we call "limpets" have descended independently from different ancestral gastropods. This general category of conical shell is technically known as "patelliform", meaning dish-shaped. Some species of limpet live in fresh water, but these are the exception. All members of the large and ancient marine clade Patellogastropoda are limpets, and within that clade the family Patellidae in particular are often called the "true limpets".

Other groups, not in the same family, are also called limpets of one type or another, because of the similar shapes of their shells. Examples include the Fissurellidae, which are known as the "keyhole limpet" family. This family is part of the clade Vetigastropoda, however, many other members of the Vetigastropoda do not have the morphology of limpets.