The mass per unit volume of a vapor; since the vapor density changes with temperature and pressure, it is commonly expressed as a specific gravity, i.e., the weight of the vapor divided by the weight of an equal volume of a reference gas (e.g., oxygen or hydrogen) at the same temperature and pressure. ... (05 Mar 2000) ... 
Type: Term Definitions: 1. the mass per unit volume of a vapor; because the vapor density changes with temperature and pressure, it is commonly expressed as a specific gravity, the weight of the vapor divided by the weight of an equal volume of a reference gas (oxygen or hydrogen) at the same temperature and pressure. &nbs...
[n] - the density of a gas relative to the density of hydrogen
vapor density Another term for absolute humidity; that is, the mass of water vapor in relation to the unit volume of space that it occupies.
vapour density noun the density of a gas relative to the density of hydrogen