Final answer:
The material placed under a porcelain restoration to mask tooth discoloration is an opacity modifier.
Step-by-step explanation:
The material placed under a porcelain restoration to mask tooth discoloration is a(n) opacity modifier. An opacity modifier is a type of dental material that is used to alter the color or transparency of a dental restoration, such as a porcelain crown or veneer, to match the surrounding natural teeth. It is applied between the tooth structure and the porcelain material to block or reduce the appearance of any discoloration.