Slug is a general term for a gastropod mollusc which has no shell, or just a small internal shell.

Slugs belong to several different families which also include snails with shells. Snails are gastropods with a coiled shell large enough for the animal to pull back inside. The families of land slugs are not very closely related, despite looking similar. The shell-less slug type is an example of convergent evolution, and so the category "slug" is not a taxonomic category.