1.(Zool.) An organ of a fish, consisting of a membrane supported by rays, or little bony or cartilaginous ossicles, and serving to balance and propel it in the water. Note: Fishes move through the water chiefly by means of the caudal fin or tail, the principal office of the other fins being to balance or direct the body, though they are also, to a certain extent, employed in producing motion.
2.(Zool.) A membranous, finlike, swimming organ, as in pteropod and heteropod mollusks.
4.A finlike appendage, as to submarine boats.
5.(Aeronautics) A fixed stabilizing surface, usually vertical, similar in purpose to a bilge keel on a ship.
