Proportion is the relationship of one part of a whole to other parts
In art it has usually meant a preoccupation of artists with finding a mathematical formula for the perfect human body.
At the time of the Renaissance, Leonardo da Vinci and Albrecht Dürer attempted to find a formula that would enable the body to be exactly inscribed in a square or a circle. Their system seems to have been to first make the height the same as the full width of the outstretched arms, and then to add to the height so that the total height was equal to eight heads. Renaissance researches into proportion were inspired by the ancient Roman writer of a treatise on architecture, Vitruvius.
A more general formula for perfect proportion is the Golden Section or Golden Mean. This is defined as a line divided so that the smaller part is to the larger part as the larger part is to the whole. It works out at roughly 8:13 or a bit over one third to two thirds. In one way or another the Golden Section can be detected in most works of art. It so named because it was considered to have some special aesthetic virtue in itself.