se ‧ le ‧ ni ‧ um /səˈliːniəm, sɪˈliːniəm/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Language: Modern Latin ; Origin: Greek selene 'moon' ]
a poisonous chemical substance, used in electrical instruments to make them sensitive to light. It is a chemical ↑ element : symbol Se