1) Something such as a chemical substance, organism, or natural force that causes an effect.
From L. agentem (nominative agens, genitive agentis), pr.p. of agere "to set in motion, drive, lead, conduct," → act.
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 kwer- "to do, to make") + -gar, → -or.
A chemical substance that, because of the reactions it causes, is used to detect, measure, or prepare another substance.
From re(act) + → agent.