What is Base Period?

Legal Definition
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.
-- Wikipedia
Legal Definition
The previous period that new data is compared against. On an index they are given arbitraty value of 100.