Definition of Rosetta Stone noun from the Oxford Advanced American Dictionary

 

Rosetta Stone

 noun
noun
NAmE//roʊˈzɛt̮ə ˌstoʊn//
 
[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.
See the Oxford Advanced Learner's Dictionary entry: Rosetta Stone