a basalt slab discovered in 1799 at Rosetta, dating to the reign of Ptolemy V (196 bc) and carved with parallel inscriptions in Egyptian hieroglyphics, demotic characters, and Greek, which provided the key to the decipherment of ancient Egyptian texts
Rosetta stone in American English
(roʊˈzɛtə)
a tablet of gray granite found in 1799 at Rosetta, a town in Egypt: because it bore parallel inscriptions in Greek and in ancient Egyptian demotic andhieroglyphic characters, it provided a key to the deciphering of ancient Egyptian writing