The word "nudibranch" comes from the Latin nudus, naked, and the Greek brankhia, gills.

Nudibranchs are often casually called "sea slugs", a non-scientific term which has led some people to assume that every sea slug is a nudibranch. However, while it is true that nudibranchs are very numerous in terms of species, and are often very attractive, there are numerous other kinds of sea slugs belonging to several taxonomic groups that are not very closely related to nudibranchs. A fair number of these other sea slugs are colorful and thus even more easily confused with nudibranchs.