The industrial Brinell hardness test has been in common use for over 100 years. The test is defined by standardized procedures stating that the Brinell hardness value is proportional to the test force divided by the surface area of the indentation. The test procedures require that the surface area be determined by measuring the indentation diameter after removing the test force. This measurement is usually made using an optical microscope, but without having a physical definition of the indentation edge. This paper proposes a physical definition of the indentation edge such that the Brinell indentation diameter can be unambiguously measured.<br />