The period of a decimal expansion is the length of the smallest block of repeating digits in the decimal expansion of a rational number that does not terminate. For example:

1/3 = 0.333333333333...; repeating block = 3, period = 1
5/7 = 0.71428571428571...; repeating block = 714285, period = 6
89/26 = 3.4230769230769...; repeating block = 230769, period = 6