Forecasting terminology
One terminology defines the period on which data are used to base the forecast as the span of the forecast (because, like a bridge, it is supported by empirical data at both ends). It then defines the future period for which forecasts are made as the gap, and the entire period of time involved in the forecast, from the start of the span to the end of the gap, as its range. Other writers refer to the base period instead of the span or variously use the terms horizon, span, or projection period to refer to the interval that was just termed the gap.
Terminology used in population forecasting
Similarly, some authors use the terms base year and launch year to refer to the earliest and most recent years from which estimates are used as a basis for forecasting vital rates and the term target year to refer to the endpoint of the projection. Other authors use the term jump-off year instead of launch year, while yet others refer to the launch year, as just defined, as the base year of the projection.
Thus, when reading the literature on population forecasts, it is important to check how each author has defined their terminology.