Byrd rule
A Senate rule that strikes provisions from a reconciliation bill when they are not primarily budgetary.
Named for Senator Robert Byrd, the rule defines several tests under which a provision counts as extraneous to a reconciliation bill: it produces no change in outlays or revenues, its budgetary effect is merely incidental to a non-budgetary policy change, it falls outside the reporting committee’s jurisdiction, or it increases the deficit beyond the budget window.
Provisions are challenged in advance in a process informally called a Byrd bath, adjudicated by the Senate parliamentarian. Waiving the rule takes sixty votes, so in practice a successful challenge removes the provision.
It is why reconciliation bills are shaped the way they are. A provision ruled 'extraneous' comes out unless sixty senators vote to keep it — which defeats the point of using reconciliation.