he ‧ li ‧ um /ˈhiːliəm/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Language: Greek ; Origin: helios 'sun' ; because it was discovered in the sun's spectrum ]
a gas that is lighter than air and is used to make ↑ balloon s float. It is a chemical ↑ element : symbol He