Pronunciation: (rē'jun),[key] —n. 1. an extensive, continuous part of a surface, space, or body: a region of the earth. 2. Usually, regions. the vast or indefinite entirety of a space or area, or something compared to one: the regions of the firmament; the regions of the mind. 3. a part of the earth's surface (land or sea) of considerable and usually indefinite extent: a tropical region. 4. a district without respect to boundaries or extent: a charming region in Connecticut. 5. a part or division of the universe, as the heavens: a galactic region. 6. a large indefinite area or range of something specified; sphere: a region of authority. 7. an area of interest, activity, pursuit, etc.; field: studies in the region of logic. 8. an administrative division of a city or territory.
9. Zoogeog.a major faunal area of the earth's surface, sometimes one regarded as a division of a larger area.
10. Anat.a place in or a division of the body or a part of the body: the abdominal region. 11. Math. a. Also called domain. an open connected set.
b. the union of such a set and some or all of its boundary points.