vapor density

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The ratio of the weight of a given volume of one gas to the weight of an equal volume of another gas, typically hydrogen, at the same temperature and pressure.

GNU Webster's 1913

1. n. (Chem.) the relative weight of gases and vapors as compared with some specific standard, usually hydrogen, but sometimes air. The vapor density of gases and vaporizable substances as compared with hydrogen, when multiplied by two, or when compared with air and multiplied by 28.8, gives the molecular weight.

WordNet 3.0

1. n. the density of a gas relative to the density of hydrogen