fin pectoral fin 2010-05-05T08:10:42Z BTO:0004653 BrendaTissueOBO In fish, one of the pair of fins that are situated one on each side of the fish just behind the gills. Normally they are used for balancing and braking, but in some species, e.g. Exocoetidae, flying fish, the extra-large fins are used for jumping and for gliding over the water surface. mgr