Slug is a common non-scientific word which is most often applied to any gastropod mollusk whatsoever that has a very reduced shell, a small internal shell, or no shell at all. Gastropods with coiled shells that are big enough to retract into are called snails.

A slug-like body is an adaptation which has occurred many times in various groups of snails, both marine and terrestrial, but the common name "slug" is most frequently encountered as applied to air-breathing land species, including a few agricultural and horticultural pest species. This article is primarily about air-breathing (pulmonate) land slugs.

Evolutionarily speaking, the loss or reduction of the shell in gastropods is a derived characteristic; the same basic body design has independently evolved many times, making slugs a strikingly polyphyletic group. In other words, the shell-less condition has arisen many times in the evolutionary past, and because of this, the various different taxonomic families of slugs, even just of land slugs, are not closely related to one another, despite a superficial similarity in the overall form of the body.

The word "slug" or "sea slug" is also used for many marine species, almost all of which have gills. The largest group of marine shell-less gastropods or sea slugs are the nudibranchs. There are in addition many other groups of sea slug such as the heterobranch sea butterflies, sea angels, and sea hares, as well as the only very distantly related, pelagic, caenogastropod sea slugs, which are within the superfamily Carinarioidea. There is even an air-breathing sea slug, Onchidella.

Category: Image - :Laevecaulis-2.jpg|thumb|right|150px|Tropical leatherleaf slug, Laevicaulis alte