Definition and background:
Italian opera of the 18th and 19th centuries that was either heroic or tragic. serious opera, as opposed to opera Buffa. Refers to 18th-century Italian operas with plots often taken from mythology or ancient history. The hero was often played by a castrato.
For more information on the music term "Opera seria" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary