base period

A particular time period whose data is used for comparative purposes when measuring economic data of other periods.

Investopedia Says:
Think of this as a yardstick for economic data. For example, if a price index has a base year of 1990, current prices are being compared to prices in that time period.

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Particular time in the past used as a yardstick when measuring economic data. A base period is usually a year or an average of years; it can also be a month or other time period. The U.S. Rate of Inflation is determined by measuring current prices against those of a base period; for instance, the Consumer Price Index is determined by comparing current prices with prices in the base reference years of 1982-1984 and the Producer Price Index is determined by comparing current prices with prices in the base reference year of 1987.

A point of time that serves as a benchmark for reflecting the change in an Index. The base period is usually set equal to 1, 100%, or 100.
Example: The Consumer Price Index set the period 1982-1984 as the base period, equal to 100. If the index is 180 in 2002, it will have risen 80% from the base period.

(DOD) That period of time for which factors were determined for use in current planning and programming.