(uncountable) The second lightest chemical element (symbol He) with an atomic number of 2 and atomic weight of 4.002602, a colorless, odorless and inert noble gas.

plural of helium

Speech whose timbre (but not pitch) has been affected by the inhalation of helium, due to the difference in the speed of sound in that gas, compared with air.

(physics) The liquified form of helium that does not solidify even at absolute zero; it shows unusual properties, such as superfluidity, due to quantum effects.

(astronomy) The sudden beginning of the burning of helium in the core of stars of intermediate mass or on the surface of an accreting white dwarf.

(physics) nuclear fusion involving helium nuclei.

(archaeology) a method of determining the age of rock that depends on the production of helium during the decay of various radioactive isotopes of uranium and thorium.

(physics, astronomy) the nucleosynthesis process, in stars with temperatures over 10⁸K, in which helium is converted to carbon and then to oxygen, neon and magnesium

(astronomy) A star in which the strength of the helium absorption lines vary periodically; at times appearing to be a helium star.

(astronomy) A star which has lost most or all of its hydrogen, leaving just a core of helium.