Eye of Providence

symbol of all-seeing eye (usually shown in triangle and/or surrounded by rays of glory)
Eye of providence, near Aaachen cathedral, in Aachen.

The eye of providence is a symbol that is often used in a relgious context. Very often, it is shown as a single human eye, surrounded by rays of light. It is often seen as a symbol for knowledge, or for a god that "sees all". In a Christian context, it is often inside a triangle, where the triangle represents the Trinity. Freemasons also use the symbol, there it stands for knowledge.