If we take any rational number and write it as a decimal, then its decimal expansion eventually repeats. If the expansion ends in repeating zeros (or nines), we say it is terminating (examples: 1 = 1.000... = 0.999... and 1/20 = 0.0500... = 0.0499...). Otherwise, the length of the (smallest) block of repeating digits is the period. For example:
Notice that if x repeats with period n, then (10n-1) x has a terminating expansion, so there is a non-negative integer m such that 10m (10n-1) x is an integer. This shows that x is rational. When x = 1/k for some integer k, it also shows that the period of 1/k is the same as the order of 10 modulo k. In particular the period of 1/k always divides Euler's phi function of k, and the period of 1/p for a prime p always divides p-1.
See Also: PeriodOfAPrime