1. A condition of displaying, revealing, exhibiting, or making accessible. 2. In dentistry, loss of hard tooth structure covering the dental pulp due to caries, dental instrumentation, or trauma. 3. Proximity or contact with a source of a disease agent in such a manner that effective transmission of the agent or harmful effects of the agent may occur. 4. The amount of a factor to which a group or individual was exposed; in contrast to the dosage, the amount that enters or interacts with the organism.