"Alla breve" in music

In short, it is a style of writing that refers back to Renaissance polyphony, where the tactus (the pulse) symbolized by the "cut C" corresponds precisely to the minim. By transcribing the original values of 15th and 16th-century music, one obtains measures consisting of two semibreves, i.e., one minim. This practice is also found in much choral music, especially sacred music, of subsequent centuries.

See, for example, Variation 22 from Bach's Goldberg Variations (p. 24 of the pdf) which is titled precisely "alla breve": it indicates that every measure must be thought of (and performed) not "in two", but "in one": it is as if every two measures in 2/2 should be grouped into a single measure in 2/1.

http://javanese.imslp.info/files/imglnks/usimg/2/2d/IMSLP74598-PMLP02982-Goldberg_Variations__facsimile_.pdf