i ‧ on /ˈaɪən $ ˈaɪən, ˈaɪɑːn/ BrE AmE noun [countable] technical
[ Date: 1800-1900 ; Language: Greek ; Origin: present participle of ienai 'to go' ]
an atom which has been given a positive or negative force by adding or taking away an ↑ electron ⇨ proton