CBO baseline
The Congressional Budget Office's projection of spending, revenue and debt under current law.
The baseline answers a deliberately narrow question: if current law does not change, what would spending and revenue be? That constraint is what makes it useful as a yardstick — every proposal can be scored against the same reference.
It also makes the baseline a poor prediction. Current law includes provisions scheduled to expire that Congress has extended every time before, and it assumes no new legislation. When a projection is described as showing what “will” happen, the assumptions have usually been dropped somewhere in the retelling.
Nearly every ten-year cost figure in budget coverage is measured against it. It is a benchmark, not a forecast of what will happen.