Antlers are boney structures unique to cervids. Antlers most commonly grow in pairs, and each antler originates out of an attachment point on the animal's skull. When the antler begins to grow, it is covered by a soft, plush layer called velvet. This velvet is vascular, and supplies oxygen and nutrients to the growing antler. Antlers grow faster than any other bone in any other mammal. Once an antler reaches its full and complete size, the velvet falls off exposing the dense textured bone. The dead bone structure is what we call "mature antler".
Below is an image of an antler with its velvet outer layer.