A cappella means sung with no instruments at all. Sing a cappella.
Without instruments a choir drifts in pitch over a long piece, which is the whole difficulty. The Italian means in the manner of the chapel, where instruments were not used.
1adjectivea cappellaSung with no instruments at all.
“The last verse was sung a cappella and the hall went silent.”
a cappella is a manner.
Italian a cappella means in the manner of the chapel, from the unaccompanied singing of the Sistine Chapel.
Italian
a cappella — in the manner of the [Sistine] chapel
English, today
a cappella
Borrowed from Italian a cappella (“in the manner of the [Sistine] chapel”), referring to non-instrumental choirs.
Etymology from Wiktionary, CC BY-SA
Practised in Music & Instruments