This standard covers the fundamental quantities for describing random instabilities of importance to frequency and time metrology. Quantities covered include frequency, amplitude, and phase instabilities; their power spectral densities; and their time-domain variances and deviations. In addition, recommendations are made for the reporting of measurements of such instabilities, especially about the recording of experimental parameters, experimental conditions, and calculation techniques. The document includes appendices covering: basic concepts and definitions; time prediction; random and systematic uncertainties; confidence limits when estimating instabilities from a finite data set; translation between the frequency domain and time domain instability measures; and an extensive bibliography of the relevant literature.