noun
1. : a clause exempting a class of persons (as from a regulatory law) because of circumstances applying before the clause takes effect ; specifically : a provision inserted in the constitutions of some southern states after the Civil War requiring high standards of literacy and substantial property qualifications of voters except for descendants of men voting before 1867
2. : one of numerous limitations on certain government regulations ; specifically : a provision by which an interstate carrier in bona fide operation prior to the effective date of regulation need not prove public convenience and necessity before it can be certified