An instrument which can separate selected ion beams with fields which do not vary with time. The fields are generally both electric and magnetic.
Sources:
Orange Book, 2nd ed., p. 202 (https://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 1546 (https://doi.org/10.1351/pac199163101541)