realm

frontier

study, subject

territory, turf

occupation, profession, pursuit, racket, vocation

ambit, amplitude, breadth, compass, confine, dimension(s), extent, ken, reach, scope, sweep, width

subfield, subspecialty

gamut, spectrum, spread

bailiwick, circle, demesne, department, discipline, domain, element, fief, fiefdom, field, province, region, specialty, sphere, terrain

frontier

horizon, panorama