orchestra (n.) Look up orchestra at Dictionary.com
c. 1600, "area in an ancient theater for the chorus," from Latin orchestra, from Greek orkhestra, semicircular space where the chorus of dancers performed, with suffix -tra denoting place + orkheisthai "to dance," intensive of erkhesthai "to go, come," from PIE root *ergh- "to rise, mount."

The PIE root is also the source of Sanskrit rghayati "trembles, rages, raves," rnoti "rises, moves," arnah "welling stream;" Old Persian rasatiy "he comes;" Hittite arai- "to rise, arise; lift, raise," hardu- "brood, descendance;" Armenian y-arnem "to rise;" Greek ornynai "to rouse, start;" Latin oriri "to rise," origo "a beginning;" Gothic rinnan, Old English irnan "to flow, run."

In ancient Rome, orchestra referred to the place in the theater reserved for senators and other dignitaries. Meaning "group of musicians performing at a concert, opera, etc." first recorded 1720; "part of theater in front of the stage" is from 1768.