A nudibranch is a member of Nudibranchia, a group of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms. There are more than 3,000 described species of nudibranchs.
The word "nudibranch" comes from the Latin nudus, naked, and the Greek βραγχια, brankhia, gills.
Nudibranchs are often casually called sea slugs, but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs, such as the colorful Aglajidae, are often confused with nudibranchs.