U.S. History. a clause in the constitutions of some Southern states after 1890 intended to permit whites to vote while disfranchising blacks: it exempted from new literacy and property qualifications for voting those men entitled to vote before 1867 and their lineal descendants.
any legal provision that exempts a business, class of persons, etc., from a new government regulation that would affect prior rights and privileges.
British Dictionary definitions for grandfather clause
(US, history) a clause in the constitutions of several Southern states that waived electoral literacy requirements for lineal descendants of people voting before 1867, thus ensuring the franchise for illiterate White people: declared unconstitutional in 1915
a clause in legislation that forbids or regulates an activity so that those engaged in it are exempted from the ban