A phenomenon resulting from quantization in the magnetic field of electrons or atoms or molecules or nuclei exposed to an external magnetic field, by which a narrow frequency band of radio waves may be absorbed and re-emitted by the object.

Absorption or emission of electromagnetic energy by nuclei in a (static) magnetic field after excitation by suitable Radio Frequency (RF) radiation : the frequency of resonance is given by the Larmor equation.

the absorption or emission of energy by atomic nuclei in an external magnetic field after the application of RF excitation pulses using frequencies which satisfy the conditions of the Larmor equation.