antenumber

from The Century Dictionary.

• noun A number one less than a given number: used, in the case of objects arranged in periods (as, for example, days are in weeks), to express the fact that the number of objects in a period is one less than the number which, in counting the objects, falls upon an object corresponding to the first: thus, 7 is the antenumber of the octave.

from the GNU version of the Collaborative International Dictionary of English.

• noun rare A number that precedes another.