Wikipedia
Base period
In economics, a base period or reference period is a point in time used as a reference point for comparison with other periods. It is generally used as a benchmark for measuring financial or economic data. Base periods typically provide a point of reference for economic studies, consumer demand, and unemployment benefit claims.
In public transport scheduling, the base period is the period of reduced service on weekdays that generally exists between the morning and afternoon rush hours.
Usage examples of "base period".
Thus, in politics, for example, we find that the British prime ministership has been turning over since 1922 at a rate some 13 percent faster than in the base period 1721-1922.