Definition and background:
An octave. Abbreviated as 8va bassa, 8a b. or more commonly today, 8vb. When written below a passage it indicates the passage should be performed an octave lower than written. See also ottava. Notes below the dashed line are played one octave lower than notated.
For more information on the music term "Ottava bassa" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary