Rosetta Stone \roh-ZEH-tuh-stohn\ noun
1 : a black basalt stone found in 1799 that bears an inscription in hieroglyphics, demotic characters, and Greek and is celebrated for having given the first clue to the decipherment of Egyptian hieroglyphics
*2 : one that gives a clue to understanding
"There is no reason patients shouldn't be able to understand the prescriptions they are given without a Rosetta stone." (Joe Graedon, et al, The Springfield [Mass.] Union-News, March 2, 1990)
Did you know?
We still use the word "hieroglyphics" for something difficult to decipher, but 200 years ago, the term was more literal -- it referred to an Egyptian writing system that was utterly unintelligible to modern man. So when an inscribed stone about the size of a coffee table was discovered in an Egyptian town called "Rosetta" ("Rashid" in Arabic), it changed the course of language history. The Rosetta stone, as it came to be called, held a key. Probably written by Egyptian priests in the 2nd century B.C., its hieroglyphic text repeated a text written in familiar Greek. As a result, Egyptologists were able to decipher the symbols. Today we also use "Rosetta stone" figuratively, as we have since the early 20th century, for other clue providers.
* Indicates the sense illustrated in the example sentence.