1640s, "to exert, as a thing acted upon, an opposite action upon the agent," from re- + act (v.). Chemical sense is from 1944. Related: Reacted; reacting (1610s). For sense development, see reaction. Meaning "perform again" (often re-act) is from 1650s.
She reacts negatively to everything I say
The hydrogen and the oxygen react
re-absorption
re-accustom
reach
re-acquaint
re-acquisition
react
reactant
reaction
reactionary
reactivate
reactive