Definition of protoporcelain in US English:

protoporcelain

noun

  • A type of early stoneware with a feldspathic glaze, closely resembling true porcelain but lacking its translucent qualities; especially a ceramic ware produced in ancient China under the Han dynasty (206 b.c.–a.d. 220). Also as a count noun: an item made of this.

Origin

Early 20th century. From proto- + porcelain.