Vesica piscis
The vesica piscis is a symbol made from two circles of the same radius, intersecting in such a way that the centre of each circle lies on the circumference of the other. The name literally means the sign of the fish.
The symbol is formed from the almond shaped area in the overlap between the circles, together with the upper arcs as far as the edges of a rectangle whose sides coincide with the widest points of the almond. The resulting figure looks both like a stylised fish, and a flattened greek alpha.
It was used by many mystical religions, such as that of Pythagoras, who considered it a holy figure. The mathematical ratio of its height to width was considered by such groups as 153:265, this was thought of as a holy number called the measure of the fish. The number appears in the Gospel of John as the exact number of fish Jesus causes to be caught in a miraculous catch of fish, and is thought by some to be a coded reference to Pythagorean beliefs.
Other uses of the shape include that by some early religions of the almond shaped central area as a representation of the female genitals, and use by early Christians under the name Ichthys.