Douglas Harper's Etymology Dictionary
1876, earlier alla capella (1847), from Italian, "in the manner of the chapel," literally "according to the chapel," from cappella "chapel" (see chapel). Originally in reference to older church music (pre-1600) which was written for unaccompanied voices; applied 20c. to unaccompanied vocal music generally.
Wiktionary
a. (alternative spelling of a cappella English) adv. (alternative spelling of a cappella English)
Wikipedia
"a capella" is a song and music video by the Norwegian duo Ylvis. It parodies a cappella groups like Pentatonix. Ylvis revealed "a capella" on I kveld med YLVIS on TVNorge on January 5, 2016. Three days after the song's release, it had 250,000 views.
Usage examples of "a capella".
Dennis and I had shared a single year in the same a capella group in college, his senior year, my freshman year.