~~
4. Triquetra
Wikimedia Commons
Also known as a “trefoil knot," triquetras are a staple of Celtic and early Christian art (sometimes drawn interlaced with a circle).
Etymology: Latin, triquetrus: three-cornered
Source: http://mentalfloss.com/article/54639/12-shapes-you-didnt-know-had-names (check out the other shapes)