Final answer:
The material placed under a porcelain veneer to mask tooth discoloration is an opaque liner.
Step-by-step explanation:
The material that is placed under a porcelain veneer to mask tooth discoloration is the opaque liner. An opaque liner is a thin layer of material that is applied to the tooth before the veneer is placed. It helps to block out any underlying discoloration and create a uniform appearance.
Learn more about tooth discoloration