Real Dictionary

underglaze in American English
designating colors, designs, etc. applied before the glaze is put on, as in painting porcelain
such colors, designs, etc.
adjective: applied to a piece before the piece is glazed
noun: color or decoration applied to a piece before it is glazed

underglaze in British English
adjective: applied to pottery or porcelain before the application of glaze
noun: a pigment, etc, applied in this way