Glass made opaque or translucent by adding tin oxide is called opal glass.
Tin oxide acts as an opacifying agent, scattering light within the glass and producing a milky or cloudy appearance. Depending on its composition and thickness, opal glass can range from softly translucent to nearly opaque. It became an important material in the development of opalescent stained glass during the 19th century.
Opal glass is often used to create skin tones, clouds, clothing, foliage, and other areas where a maker wants color and diffusion rather than transparent jewel-like light. Different pieces can show several colors within one sheet, a characteristic valued by American stained-glass artists.
The term should not be confused with opalescent glass as a broader artistic category, or with flashed glass, which has a thin layer of colored glass fused onto a clear body. Crown glass and pot-metal glass describe different glassmaking traditions.