re ‧ a ‧ gent /riˈeɪdʒ ə nt/ BrE AmE noun [countable] technical
[ Date: 1700-1800 ; Language: Modern Latin ; Origin: present participle of reagere 'to react' , from Latin agere 'to act' ]
a substance that shows that another substance in a ↑ compound exists, by causing a chemical ↑ reaction