This article is from the Charmed FAQ, by Colin A. McGregor (faq@mcgregor.org) with numerous contributions by others.
It's a Triquetra or "Trinity Knot". In pre-Christian times the Celts used
this symbol to represent the triple goddess (Maiden, Mother and Crone).
Early Celtic Christians used the symbol to represent the trinity of God
the Father, Jesus Christ, and the Holy Spirit.
 
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