Definition and background:
An octave. Abbreviated as 8va. When written above a passage it indicates the passage should be performed an octave higher than written, and when written below a passage it indicates the passage should be performed an octave lower than written. In modern notation, the indication 8vb ( ottava bassa) is used to designate an octave lower. See also ottava bassa.
For more information on the music term "Ottava" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary