Definition of Rosetta Stone noun from the Oxford Advanced Learner's Dictionary

 

Rosetta Stone

 noun
noun
BrE BrE//rəʊˈzetə stəʊn//
 
; NAmE NAmE//roʊˈzetə stoʊn//
 
[singular]
 
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something, especially a discovery, that helps people to understand or find an explanation for a mystery or area of knowledge that not much was known about From the name of an ancient stone with writing in three different languages on it that was found near Rosetta in Egypt in 1799. It has helped archaeologists to understand and translate many other ancient Egyptian texts. Culture The same text is written on the Rosetta Stone three times in three different types of writing, including hieroglyphics (= Egyptian writing). This enabled the French language expert Jean-François Champollion (1790-1832) to read hieroglyphics for the first time.
See the Oxford Advanced American Dictionary entry: Rosetta Stone