What is "mackerel shark"

Porbeagle \Por"bea`gle\, n. (Zo["o]l.) A species of shark ( Lamna cornubica), about eight feet long, having a pointed nose and a crescent-shaped tail; -- called also mackerel shark. [Written also probeagle.]

n. 1 Any shark of the order Lamniformes, having two dorsal fins an anal fin, five gill slits and eyes without nictitating membrane 2 Any shark of the family Lamnidae.

n. fierce pelagic and oceanic sharks