/hee"lee euhm/ , n.
an inert, gaseous element present in the sun's atmosphere and in natural gas, and also occurring as a radioactive decomposition product, used as a substitute for flammable gases in dirigible balloons. Symbol: He; at. wt.: 4.0026; at. no.: 2; density: 0.1785 g/l at 0°C and 760 mm pressure.
[ 1875-80; héli ( os ) the sun + NL -ium -IUM ]