The deflection of an ion beam as a result of the motion of the ions in a magnetic field (magnetic sector). Generally the direction of motion of the ions is at right angles to the direction of the magnetic field, and the motion is uniform.
Orange Book, 2nd ed., p. 201 (http://media.iupac.org/publications/analytical_compendium/)
PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1544 (https://doi.org/10.1351/pac199163101541)