What is "vesica piscis"

Vesica \Ve*si"ca\, n. [L.] A bladder.

Vesica piscis. [L., dish bladder.] (Eccl. Art) A glory, or aureole, of oval shape, or composed of two arcs of circles usually represented as surrounding a divine personage. More rarely, an oval composed of two arcs not representing a glory; a solid oval, etc.

The vesica piscis is a shape that is the intersection of two disks with the same radius, intersecting in such a way that the center of each disk lies on the perimeter of the other. The name literally means the "bladder of a fish" in Latin. The shape is also called mandorla (" almond" in Italian).

This figure appears in the first proposition of Euclid's Elements, where it forms the first step in constructing an equilateral triangle using a compass and straightedge. The triangle has as its vertices the two disk centers and one of the two sharp corners of the vesica piscis.