"a region, district, land, country," late 14c., from Old French plage (13c.) and directly from Late Latin plagia "a plain, shore," noun use of adjective (plagia regio), from plaga "a region, stretch of country" (see pelagic). From early 15c. as "one of the four cardinal directions of the compass." Astronomical use in reference to a region of the sun's chromosphere is from 1949.