Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the earth's surface. They are also called mother-of-pearl clouds.
WRCC Climate and Weather Terms Glossary

Nacreous Clouds - Clouds that occur in the stratosphere at altitudes above 20 km and are usually iridescent and luminous in color. They may also be called mother of pearl clouds.
[Science; v264: page 527; 1994.]
[Webster's Third new International Dictionary; Third Edition: page 1499; 1986.] 
Atmospheric Chemistry Glossary