Press–Schechter formalism
The Press–Schechter formalism is a mathematical model for predicting the number of objects (such as galaxies, galaxy clusters or dark matter halos) of a certain mass within a given volume of the Universe. It was described in an academic paper by William H. Press and Paul Schechter in 1974.
Source: Wikipedia — Press–Schechter formalism (CC BY-SA 4.0)