A circle is a simple shape of Euclidean geometry that is the set of all points in a plane that are at a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius. It can also be defined as the locus of a point equidistant from a fixed point.

A circle is a simple closed curve which divides the plane into two regions: an interior and an exterior. In everyday use, the term "circle" may be used interchangeably to refer to either the boundary of the figure, or to the whole figure including its interior; in strict technical usage, the circle is the former and the latter is called a disk.

A circle can be defined as the curve traced out by a point that moves so that its distance from a given point is constant.

A circle may also be defined as a special ellipse in which the two foci are coincident and the eccentricity is 0.

The word "circle" derives from the Greek κίρκος (kirkos), itself a metathesis of the Homeric Greek κρίκος (krikos), meaning "hoop" or "ring". The origins of the words "circus" and "circuit" are closely related.

In religious symbolism, the circle represents the spirit and the cosmos. It is said that the circle itself is constructed from "some thing" (the unbroken line) and "no thing" (the space inside and outside of the line). Therefore, the circle unifies spirit and matter.

The structure of the circle has great strength—think of the cylindrical shape of a lighthouse, built that way in order to withstand the fiercest attack of the sea.

They physical and spiritual strength of this symbol are there because the perfect circle has no beginning and no end; it is unassailable. This power is the reason why the circle is used in magical practices such as spell-casting. The magic circle creates a fortress of psychic protection, a physical and spiritual safe haven where unwanted entities cannot enter.

Hermes Trismegistus said of the circle: "God is a circle whose center is everywhere and circumference is nowhere."

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