Dentin is a hard tissue that lies beneath the enamel in a tooth, providing support and protection to the inner structures. It is not as hard as enamel but harder than pulp, the innermost part of the tooth.
Tooth structure includes enamel, dentin, pulp, and cementum. Enamel is the hard outer layer, dentin makes up most of the tooth, pulp is the innermost part containing nerves and blood vessels, and cementum covers the tooth roots.
It is the enamel, usually. There could be dentin exposed if there is decay.
It's Dentin
The soft innermost part of a tooth, containing blood vessels and nerves.
enamel found only in the crown of the tooth ( the exposed part of the tooth ), dentin found in both the crown and the root of the tooth ( root : the embedded part of the tooth ) and cement which hold the tooth in it's place
The part of the tooth that is covered with enamel is the crown.
The part of a tooth that is exposed and above the gum is called "crown", not to confuse with a crown that is prothesis to restore a heavily damaged tooth. The part of a tooth that is hidden inside the bone is called "root".
I believe what you are refering to is known as the nerve, also refered to as dental pulp or pulp chamber. This is found deep inside the tooth under the dentin which is under the enamel (located on top of the tooth). The cementum is the outer layer that protects the dentin on the lower half of the tooth.
The dentinoenamel junction is the area where the hard enamel on the outside of the tooth meets the softer dentin on the inside. This junction is more sensitive because it lacks the protective enamel layer that covers the rest of the tooth, making it more susceptible to external stimuli like temperature changes and acids that can cause sensitivity and pain.
Dentin is the substance making up the main part of a tooth, while the enamel is the substance covering the the visible part of an anatomical crown in a thin layer. Inside the tooth, well protected by the dentin, is the pulp chamber and the root canals - the nerve of the tooth.
Enamel (outer part, hard), dentin (under enamel), pulp (center, where most nerves are), cementum (covers root of tooth)