Modulo

From the Quicksilver Metaweb.

To add (subtract, etc.) several numbers modulo 2, or 3, or any other natural number n, is to add (subtract, etc.) as if one is counting around a "clock" face. Imagine the clock has numbers to 7. If one was to point to 2, then add 9 to that, one would arrive at 4. Thus, 2 + 9 modulo 7 = 4.