rosetta stone(n = noun.communication) - a part of an inscribed granite stela that was originally about six feet tall and was set up in 196 BC; the inscriptions in hieroglyphics and Demotic and Greek gave the first clues to the decipherment of Egyptian hieroglyphics;
is a kind of stela, stele

rosetta stone, n. a key to previously unattainable understanding.

Etymology:
a stone found near Rosetta in Egypt, with a trilingual inscription of the 2nd c. BC in Egyptian hieroglyphs, demotic, and Greek, important in the decipherment of hieroglyphs

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