A gastropod whose shell is either absent or internal.
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Orthogastropoda
Orthogastropoda includes snails, which have external shells, and slugs, which do not. The absence of a shell in Orthogastropoda has evolved independently multiple times, meaning that there is more than one scientific classification for animals commonly lumped together as slugs.
Sea slugs are order Opisthobranchia, suborder Nudibranchia.
Most Land slugs are order Pulmonata, suborder Stylommatophora.
They are rather cute and slimy as mollusks go, but they have a bad reputation because people plant stuff that they like to eat. Whose fault is this?
In the imperial system of measurement a slug is a technical measurement of mass. It is equal to the mass which when acted on by one pound force acquires an acceleration of one foot per second per second. It is equal to 32.174 pounds mass in a standard gravity.
In the metric system, we use the kilogram, and everything is much more simple.