Goldschmidts tolerance factor (from the German word Toleranzfaktor') is an indicator for the stability and distortion of crystal structures. It was originally only used to describe the perovskite ABO structure, but now tolerance factors are also used for ilmenite.
Alternatively the tolerance factor can be used to calculate the compatibility of an ion with a crystal structure.
The first description of the tolerance factor for perovskite was made by Victor Moritz Goldschmidt in 1926.