definition of rosetta stone 1 a black basalt stone found in 1799 that bears an inscription in hieroglyphics demotic characters and greek and is celebrated for having given the first clue to the decipherment of egyptian hieroglyphics2 one that gives a clue to understandingefinition of rosetta stone 1 a black basalt stone found in 1799 that bears an inscription in hieroglyphics demotic characters and greek and is celebrated for having given the first clue to the decipherment of egyptian hieroglyphics