Dentine is the hard material that most of a tooth is made of. It sits under the white outer coat you can see when you smile — that white coat is enamel — and it is thicker than enamel, a little yellow, and full of tiny tubes. People call it a kind of bone, and that is close enough for everyday talk, though actually it is not quite the same as the bone in your arm: it is its own hard living tissue, made by the tooth itself. If a hole or wear goes through the enamel, the dentine is what starts to hurt, because those tubes lead in toward the soft living part in the middle. Dentists use this word all the time; ordinary people usually just say 'the tooth' and never name the layer. In American spelling you will often see dentin, without the last e. Same thing.