Rosetta Stone Definition

rō-zĕtə

noun

A tablet of gray granite found in 1799 at Rosetta, a town in Egypt: because it bore parallel inscriptions in Greek and in ancient Egyptian demotic and hieroglyphic characters, it provided a key to the deciphering of ancient Egyptian writing.

Webster's New World

pronoun

A large inscribed stone found near Rosetta, Egypt, in 1799; later used as a basis for understanding many previously undecipherable examples of hieroglyphic writing.

Wiktionary

(hence) Any source of information crucial to interpreting.

My friend’s carefully-compiled notebook was the Rosetta Stone that opened understanding of the physics lectures.

Wiktionary