'Fish' is a folk-biological term that does not correspond precisely to any monophyletic taxonomic group. This can be appreciated by noted that a coelacanth is more closely related to a human than to a tuna, or that a tuna is more closely related to a human than it is to a shark. I.e., some things that are intuitively fish are more closely related to non-fish than to other fish. Basically, the folk term 'fish' refers to all vertebrates other than tetrapods, although it is somewhat ambiguous in regards to animals such as sea-horses, eels, hagfish and sting-rays.