Dictionary.net

[It.] In Mus., = the notes individually to be made shorter, i.e. the pace to be quicker than usual. It is a kind of common time marked used in church music, each bar being = a breve = 2 semibreves = 4 minims, but the minims being played as if they were crotchets. The division of the bar into two parts each = two minims is called alla cappella time. But the use of the term is not always clear.