reactant (n.)

1640s, "to exert, as a thing acted upon, an opposite action upon the agent," from re- "back" + act (v.). Related: Reacted; reacting (1610s). For sense development, see reaction. The verb meaning "to perform again, do a second time" (often written re-act and given full pronunciation of the prefix to distinguish it from react) is from 1650s, from the "again" sense in re-.

agent or instrumental suffix, from Old French and French -ant, from Latin -antem, accusative of -ans, present-participle suffix of many Latin verbs. Compare -ance.