What’s the difference between a nudibranch and a sea slug?

Until I read a gorgeous new book on nudibranchs by Tim Nicholson I didn’t think there was a difference. Not so. All nudibranchs are sea slugs but not all sea slugs are nudibranchs.

Sea slug is the common name for any marine gastropod without a visible shell (slug-like rather than the snail-like) including sea hares & nudibranchs. A nudibranch is 1 type of gastropod among many.

Book review is at https://news.scubatravel.co.uk/whats-the-difference-between-a-nudibranch-and-a-sea-slug.html