Diameter of a collection of different-size spherical objects such as particles, droplets, and bubbles can be represented by a single value called the mean diameter. There are many ways of calculating and expressing the mean diameter of particulate matter. One of them is the Sauter mean diameter also called the surface-volume mean diameter. This diameter has been extensively used in numerous publications without paying attention to its physical meaning. In this article, the physical meaning of the Sauter mean diameter of spherical objects was derived and presented. The Sauter mean diameter of a collection of spherical objects of different sizes is equal to the diameter of equisized spherical objects forming a collection. The polysized and equisized systems have different numbers of spherical objects, identical total surface area, and identical total volume. If the surface energy of all spheres of both systems is the same, they can be called equienergetic.