Triquetra.

An ancient and powerful symbol of Unity brought to the attention of the public by the series "Charmed". (While some didn't like the show, personally I'm in favour of anything that portrays witches as attractive, intelligent and personable "Good Guys" rather than evil hags with hook noses and hairy warts.)

The three arcs of the Triquetra are the simplest of the Unicursal designs. Unicursal being those designs that can be drawn without lifting pen from paper and thus preventing the entrance of evil into the design and the work. This ancient Celtic design is old enough that the origins are lost in the mists of time, although stories and legends are many.

Suffice to say that like the Pentagram, the Triquetra has a special place in the hearts and minds of Pagans and witches everywhere.