Definition of "bannerfish"

A large sailfish of the species Istiophorus platypterus, of the swordfish family, with a broad banner-like dorsal fin  examples

Any of several species of butterflyfish in the genus Heniochus (order Chaetodontiformes) that superficially resemble the Moorish idol (Zanclus cornutus, order Acanthuriformes), though not closely related.  examples