Real Dictionary

Rosetta stone in American English
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
noun: a stone slab, found in 1799 near Rosetta, bearing parallel inscriptions in Greek, Egyptian hieroglyphic, and demotic characters, making possible the decipherment of ancient Egyptian hieroglyphics

Rosetta stone in British English
noun: 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