The vesica piscis is a type of lens, a mathematical shape formed by the intersection of two disks with the same radius, intersecting in such a way that the centre of each disk lies on the perimeter of the other. In Latin, "vesica piscis" literally means "bladder of a fish", reflecting the shape's resemblance to the conjoined dual air bladders ("swim bladder") found in most fish. In Italian, the shape's name is mandorla ("almond").
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 ruler. The triangle has as its vertices the two disk centres and one of the two sharp corners of the vesica piscis.