Noun
an exemption based on circumstances existing prior to the adoption of some policy; used to enfranchise illiterate whites in south after the American Civil War
Source: WordNetButch Huskey wore the number throughout the rest of his Mets career because of a grandfather clause placed on the retired number by MLB. citation Mo Vaughn also wore 42 during his stint with the Mets, because of the same clause. Source: Internet
The owner also gave up a portion of his parking lot, but the building itself would be protected by a grandfather clause requiring a certain length between a road and a building since it was a pre-existing building. Source: Internet