Minimum Detectable Activity (MDA) is an important technical characteristic of gamma spectrometers and radiometers designed to measure small amounts of radionuclides. MDA describes the instrument’s lower detection threshold. However, in many publications, as well as in Russian normative documents, there is a mixing of the notion of MDA with notions from the theory of accuracy (error) of measurements. Obviously, the error is a separate interval characteristic of the instrument, not directly related to the MDA. In the present paper an attempt is made to unambiguously and relatively simply define the concept of «MDA» on the basis of the theory of detection. Methods and formulas for determining the MDA of a gamma spectrometer or radiometer are given.