Definition and background:
A staff of five lines connected at the left end by a symbol; this symbol contains an arrow which points to the third line, indicating that that line is middle C ( c1). This clef is sometimes used for the notation of the alto part because it facilitates the printing of the part with minimal use of ledger lines. It is also known as the viola clef as the viola is the only instrument that uses it with consistent regularity. It can also be found to some degree in the music of the English horn, trombone and bassoon. See also C Clef; [Fr.] Clef d'ut; [Ger.] C-Schlussel; [It.] Chiave di tenore.
For more information on the music term "Alto clef" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary