Triquetra originally meant "triangle" and was used to refer to various three-cornered shapes. It has come to refer exclusively to a particular more complicated shape formed of three vesicae piscis (the leaf-like shape in between two equal diameter circles when both centers are on the circumference of the other circle), sometimes with an added circle in or around the three lobes. Also known as a "trinity knot" when parallel doubled-lines are in the graph, the design is used as a religious symbol adapted from ancient Pagan Celtic images by Christianity, as is common in Christianity. It is similar to Odin's symbol, the valknut.

The triquetra is often found in insular art, most notably metal work and in illuminated manuscripts like the Book of Kells. It is also found in similar artwork on Celtic crosses and slabs from the early Christian period. The fact that the triquetra rarely stood alone in medieval Celtic art has cast reasonable doubt on its use as a primary symbol of belief. In manuscripts it is used primarily as a space filler or ornament in much more complex compositions, and in knotwork panels it is a design motif integrated with other design elements. This widely recognised knot has been used as a singular symbol for the past two centuries by Celtic Christians, pagans and agnostics as a sign of special things and people that are threefold.

Source: Triquetra from Wikipedia

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