1. A tablet found near Rosetta, Egypt, in 1799. Carved with Greek, demotic, and hieroglyphic inscriptions, it became the key to deciphering ancient Egyptian writing. The discovery of the Rosetta Stone made ancient Egypt accessible to us today.

2. By extension, an important clue to understanding something that is challenging or puzzling. Don't worry, this map will be the Rosetta stone that gets us out of this cave. I've been struggling with this experiment, but these new results may prove to be the Rosetta stone that makes everything else comprehensible.