Definition and background:
A symbol used to indicate that the music notated should be performed at a pitch level either an octave higher or an octave lower than is indicated by the normal version of that clef. Octave clefs include the G octave clef ( octave treble clef) and the F octave clef ( octave bass clef). The need for these clefs comes from the instruments and voice types that read music printed on the staff (with a minimum of ledger lines), but in actuality, sound an octave higher or lower than where the notes are written. Examples would include the male tenor voice (sounding an octave lower than written) and the piccolo (sounding an octave higher than written). If this were notated correctly, the performer would be reading notation with an excessive number of ledger lines. See also octave clef; G octave clef; octave treble clef; vocal tenor clef; double treble clef; F octave clef; octave bass clef. Treble and bass clefs can also be modified by octave numbers. An eight or fifteen above a clef raises the intended pitch range by one or two octaves respectively. Similarly, an eight or fifteen below a clef lowers the pitch range by one or two octaves respectively. A treble clef with an eight below is the most commonly used, often used instead of a C clef for tenor lines in choral scores.
For more information on the music term "Octave clef" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary