Puffinus is a genus of seabirds in the order Procellariiformes. It comprises about 20 small to medium-sized shearwaters. Two other shearwater genera are named: Calonectris, which comprises three large shearwaters, and Procellaria with another four large species. The latter are usually called petrels, although they are thought to be more closely related to the shearwaters than to the other petrels. Despite the resemblance in the name, the puffins are auks, and completely unrelated to the shearwaters in the genus Puffinus; the genus name Puffinus is actually a New Latin loanword based on the English "puffin". The original Latin term for shearwaters was usually the catchall name for sea-birds, mergus. The taxonomy of this group is the cause of much debate, and the number of recognised species depends on the source.