It's Latin for carina.

The keel is the sharp edge of the belly, less often the sides of the caudal peduncle, covered with scales or bare. Skin keel can be on the sides of the caudal stem and behind the dorsal fin. In some herring fish, the keel is covered with a special shape of roof-like keel scales.

In many top-floating fish, possessing a particularly bright silvery luster, the lateral planes of the body converge on the ventral side at an acute angle, forming a longitudinal abdominal keel, giving the body of the fish some resemblance to a knife blade. Among fish, such a feature is well expressed in various species of herring and chekhoni. In the keelless fish formed a shadow at the bottom, while the fish with an abdominal keel such a shadow is not formed. Abdominal keel, enhances the protective role of the silvery shine of the lateral surfaces of the body of fish, making them virtually invisible to predators that swim directly under them.