c. 1300, regioun, "tract of land of a considerable but indefinite extent," also "a kingdom, country, nation; the people of a country," from Anglo-French regioun, Old French region "land, region, province" (12c.) and directly from Latin regionem (nominative regio) "a district, portion of a country, territory, district; a direction, line; boundary line, limit," noun of state from past-participle stem of regere "to direct, rule" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").
From late 14c. as "a part of the world," also "rural area around a city." Phrase in the region of "about" (of numbers, etc.) is attested from 1961.