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The Magen David: a hexagram (six-pointed star formed by overlapping two equilateral triangles).

The precise origin of the use of it as a Jewish symbol remains unknown, but it apparently emerged in the context of medieval Jewish protective amulets (segulot). The Jewish Encyclopedia cites a 12th-century Karaite document as the earliest Jewish literary source to mention the symbol.[1]

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1. Judaism or Jewish identity.
2. (cartography) marks a synagogue.

This shape is generically called a hexagram (which has a wider array of definitions), but this specific symbol has been labelled by Unicode for use as the Star of David.