vapour density n (General Physics) the ratio of the density of a gas or vapour to that of hydrogen at the same temperature and pressure. See also relative density Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014