Patient-friendly definition from the DentistsDirectory.org dental glossary.
Dentin is the hard, calcified tissue that makes up the bulk of a tooth's structure beneath the outer enamel. While enamel covers the visible crown of the tooth, dentin extends from the crown all the way down the root, surrounding the inner pulp chamber and root canals. Dentin is notably softer than enamel — approximately 3 on the Mohs hardness scale versus 5 for enamel — and is naturally yellowish in colour, which is why teeth appear more yellow when enamel becomes thin or wears away.
Dentin contains millions of microscopic channels called dentinal tubules that run from the outer surface to the pulp at the centre of the tooth. These tubules contain fluid and, in some models, tiny cellular extensions from the pulp. When the outer dentin is exposed — through enamel erosion, gum recession, or abrasion — changes in temperature, pressure, sweet foods, or even cold air cause rapid fluid movement in the tubules, which the nerve within the pulp interprets as pain. This is the mechanism behind dentinal hypersensitivity (tooth sensitivity).
Decay progresses far more rapidly through dentin than through enamel — partly because dentin is softer and partly because its tubules allow acid and bacteria to spread inward toward the pulp. Once decay reaches dentin, it becomes increasingly urgent to treat. Exposed root dentin (from gum recession) is also more susceptible to decay than enamel-covered surfaces, which is why good gum health and appropriate brushing technique matter for preventing root cavities.