Definition and background:
A form of Italian poetry that is set into very strict stanzas of eight hendecasyllabic lines (or eight lines with eleven syllables). This form of poetry was also known as rispetto. These stanzas were set to music by composers of the frottola and 16th-century madrigal and later, the Strambotto. The ottava rima typically is an existing poem with multiple stanzas of the rhyme scheme abababcc.
For more information on the music term "Ottava rima" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary