Tooth: One of the structures within the mouth that allow for biting and chewing. Teeth have different shapes, depending on their purpose. The sharp canine and incisor teeth allow for biting, and the flattened, thick molars in the back of the mouth provide grinding surfaces for masticating food. All teeth have essentially the same structure: a hard crown above the gum line, which is attached to two or four roots by a portion called the neck. The roots are covered with a very thin layer of bone, and they keep the tooth embedded in the bones of the jaw. The exposed exterior of the tooth is covered with tough enamel. Under the enamel is a thick layer of dentin, and in the center is the pulp, which contains blood vessels and nerves.