A time indication, equivalent to alla breve.
In church or chapel style; - said of compositions sung in the old church style, without instrumental accompaniment; as, a mass a capella, i. e., a mass purely vocal.
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Webster, Noah. Entry for 'A Cappella'. Noah Webster's American Dictionary. https://www.studylight.org/dictionaries/eng/web/a/a-cappella.html. 1828.