The term "knife fish" defines a fish with a much elongated, tapering, laterally compressed and tapering body with usually a very long anal fin and very often missing pelvic and dorsal fins. This body form has developed several times within the Teleostei, such as the Notopteridae in the Osteoglossomorpha - the Old World knife fishes -, the Gymnotoidei within the Cypriniformes, known as the (South American) knife fishes, which also have electric organs, without in any way being related to the Mormyridae. Also among the Clupeiformes, - herrings and anchovies - this body-form is to be found. Finally Gymnarchus can be regarded as a sort of knife fish, only that in it the very long dorsal fin serves as the main organ of propulsion.

Alternative form for Knife fish : gymnotoidei.