Slug is a common name for an apparently shell-less terrestrial gastropod mollusc. The word "slug" is also often used as part of the common name of any gastropod mollusc that has no shell, has a very reduced shell, or has only a small internal shell (this is in contrast to the common name snail, which applies to gastropods that have a coiled shell large enough that the animal can retract its soft parts fully into it).
Slugs exist on land and in the sea, and there is even one genus of freshwater slugs, Acochlidium. The unadorned word "slug" is however applied primarily to land slugs, whereas slugs from the sea or from fresh water are usually referred to as "sea slugs" or "freshwater slugs". Land gastropods with a shell that is not quite vestigial, but is too small to retract into (like many in the family Urocyclidae), are known as semislugs.
Various taxonomic families of slugs form part of several quite different evolutionary lineages, which also include snails. Thus, for example, the various families of land slugs are not closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is emphatically a polyphyletic one.
Slugs, like all other gastropods, undergo torsion (a 180° twisting of the internal organs) during development. Internally, slug anatomy clearly shows the effects of this rotation—but externally, the bodies of land slugs appear more or less symmetrical, except for the positioning of the pneumostome, which is on one side of the animal, normally the right-hand side. The soft, slimy bodies of slugs are prone to desiccation, so land-living slugs are confined to moist environments and must retreat to damp hiding places when the weather is dry. The subsequent information in this article applies to land slugs.