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A snail is a common name for a kind of mollusc. The term is used for a gastropod with a coiled shell. Their fossil records extends back into the Carboniferous period.

Land snails and slugs breathe with a kind of lung. They used to be put together in a group, the Pulmonata. This was a well-known order in traditional taxonomy. However, the Pulmonata is polyphyletic. This means the same life-style evolved in a number of different lines. This is called convergent evolution. Therefore, the Pulmonata is no longer an official term in biological classification.

The term "snail" is also sometimes used for aquatic snail-like gastropods, which usually have gills. Actually, most snail species are marine snails. There are more species of them, and they are far greater in numbers. Many kinds of snails can also be found in fresh water habitats.