The phrase a cappella is among the most misunderstood musical terms. The predominant, and most "correct" spelling, is ...
Musicologists have fun debating the extent to which a cappella, "in the style of the chapel," can include instrumental accompaniment. Some argue that early sacred a cappella performances would sometimes include instruments that double a human voice part. So, the correct definition of a cappella should be something like "singing without independent instrumental accompaniment."
At Primarily A Cappella, we are trying to popularize this style of music, so we like to keep it simple.