A nudibranch ˈ is a member of what is now a taxonomic clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

, and what was previously a suborder, of soft-bodied

Soft-bodied organisms

The term "soft-bodied organism" is used to describe animals without skeleton, roughly corresponding to the group Vermes as proposed by Carl von Linné. All animals have muscles, but since muscles can only pull, never push, a number of animals have developed hard parts that the muscles can pull on,...

, marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms. The clade Nudibranchia is the largest clade within the heterobranchs, with more than 3,000 described species.

The word "nudibranch" comes from the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 nudus, naked, and the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 brankhia, gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s.

Nudibranchs are often casually called "sea slug

Heterobranchia

Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....

", but many sea slugs belong to several taxonomic groups which are not related to nudibranchs.