A harpoon is a long spear that's barbed. To harpoon is to strike an animal, like a whale, with a harpoon. If you are Moby Dick, you steer clear of harpoons.
There are many weapons people use to hunt, such as guns, knives, and spears. When people hunt sea life such as whales and sharks, they use a type of spear called a harpoon. A harpoon is long like a spear, but it's also barbed, so when it penetrates an animal, it sticks in. The hunter throws the harpoon, and it has a line attached to it — much like a fishing line. Hunting animals in this way is called harpooning.