Slug is a common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 that is normally applied to any gastropod mollusc
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

 that lacks a shell, has a very reduced shell, or has a small internal shell. (This is in contrast to the common name snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

, applied to gastropods that have a coiled shell large enough that the soft parts of the animal can retract fully into it.)

Slugs belong to several different lineages which also include snails that have shells.