sailfish \ˈ ̷ ̷ˌ ̷ ̷\noun 1.: any of several large pelagic fishes constituting a genus (Istiophorus) related to the swordfish but having teeth, scales, pelvic fins of a few rays, and a very large dorsal fin highest at or behind its middle — see atlantic sailfish, pacific sailfish 2.: basking shark