Definition: A Cappella is a term applied to singing without any accompaniment. "Cappella" means "chapel" in Italian, and therefore, means "in the manner of the chapel." There are many fantastic a Cappella works. Here are a just a few of my favorites a Cappella works via these YouTube recordings.
Samuel Barber's Angus Dei (View on YouTube)
Eric Whitacre's Lux Aurumque (View on YouTube)
Ralph Vaughan Williams Kyrie from the G minor Mass (View on YouTube)
Michael Praetorius' Es ist ein Rose Entsprungen (View on YouTube)
Alternate Spellings: acappella
Common Misspellings: a capella, acapella