Source : Webster's Revised Unabridged Dictionary (1913)

Alla breve \Al`la bre"ve\ [It., according to the breve.] (Old
   Church Music)
   With one breve, or four minims, to measure, and sung faster
   like four crotchets; in quick common time; -- indicated in
   the time signature by ?.

Source : WordNet®

alla breve
     n : a musical time signature indicating two or four half notes
         to a measure