Sea lemon is a loosely applied common name for a group of medium-sized to large shell-less colorful sea slugs or nudibranchs, specifically dorid nudibranchs in the taxonomic family Dorididae and other closely related families. These are marine gastropod mollusks.

Doridoidea, commonly known as dorid nudibranchs, are a taxonomic superfamily of medium to large, shell-less sea slugs, marine gastropod mollusks in the clade Doridacea, included in the clade Nudibranchia.