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 and hieroglyphic characters, it provided a key to the deciphering of ancient Egyptian writing
See Rosetta stone in American Heritage Dictionary 4
(rō-zĕtˈə)
noun
A basalt tablet bearing inscriptions in Greek and in Egyptian hieroglyphic and demotic scripts that was discovered in 1799 near Rosetta, a town of northern Egypt in the Nile River delta, and provided the key to the decipherment of Egyptian hieroglyphics.