| 1. | 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. |
| 2. | a clue, breakthrough, or discovery that provides crucial knowledge for the solving of a puzzle or problem. |
Ro·set·ta stone  (rō-zět'ə)  n. 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. |
A stone discovered in Egypt in the late eighteenth century, inscribed with ancient Egyptian hieroglyphics and a translation of them in Greek. The stone proved to be the key to understanding Egyptian writing.
Note: A “Rosetta stone” is the key to understanding a complex problem.