An Etymological Dictionary of Astronomy and Astrophysics

1. Something such as a chemical substance, organism, or natural force that causes an effect.
   
2. Somebody who provides a particular service for another.

Etymology (EN): From L. agentem (nominative agens, genitive agentis), pr.p. of agere “to set in motion, drive, lead, conduct,” → act.

Etymology (PE): Konešgar, from koneš verbal noun of kardan “to do, to make” (Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,”
karma “act, deed;” PIE base k^wer- “to do, to make”)

• -gar, → -or.