xen ‧ on /ˈzenɒn $ ˈziːnɑːn, ˈze-/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Language: Greek ; Origin: xenos 'strange' (because it is rare) ]
a colourless gas that is found in very small quantities in the air. It is a chemical ↑ element : symbol Xe