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In physics, optically detected magnetic resonance (ODMR) is a technique for detecting quantum systems with spin-dependent optical transitions. In the case of fluorescent point defects in crystals, the "ODMR signal" refers to a change in the defect's fluorescence intensity due to a simultaneously applied AC magnetic field. When the magnetic field frequency is resonant with a spin transition, which means that the frequency matches the energy splitting between two spin states, the spin system undergoes Rabi oscillations between the two states. If the two spin states have different fluorescence intensities, this results in a change in the defect's fluorescence. This can be used to measure properties of the spin such as the zero-field splitting and the gyromagnetic ratio. There may be many spin transitions, and their characteristics as observed with ODMR depend sensitively on the conditions of the measurement, motivating the use of ODMR as a technique for quantum sensing.