The grey seal is a large seal of the family Phocidae, which are commonly referred to as "true seals" or "earless seals". The only species classified in the genus Halichoerus, it is found on both shores of the North Atlantic Ocean. In Latin, Halichoerus grypus means "hook-nosed sea pig". Its name is spelled gray seal in the US; it is also known as Atlantic seal and the horsehead seal.

Pinnipeds, commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae, Otariidae, and Phocidae, with 34 extant species and more than 50 extinct species described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage. Pinnipeds belong to the suborder Caniformia of the order Carnivora; their closest living relatives are musteloids, having diverged about 50 million years ago.