Answer:
Plessy v. Ferguson, (1896)
The phrase "separate but equal" derives from the Louisiana Separate Car Act of 1890, which was challenged in the US Supreme Court case Plessy v. Ferguson, (1896). The federal government allowed states to determine their own segregation laws after Reconstruction ended in 1877, and seventeen states created statutes that limited co-mingling of whites and African-Americans.
Case Citation:
Plessy v. Ferguson, 163 US 537 (1896)