An Etymological Dictionary of Astronomy and Astrophysics

A hypothetical element, the existence of which was postulated in the nineteenth century to account for unidentified emission lines
(e.g. at 3727 and 5007 Å) in the spectra of some luminous nebulae. It was also believed that this element had a small atomic weight. However, the advances of chemistry and physics showed that all the light elements were known and there was no place for this elusive element. Those unidentified lines have now been shown to come from known elements, but they are not usually observable under laboratory conditions. → forbidden lines.

See also: From nebul(a), → nebula, + -ium L. neuter suffix.