relative density
ratio of the density (mass per volume) of a substance to the density of a given reference material
en-US/ˈɹɛlətɪv ˈdɛnsɪti/
en-GB/ˈɹɛləˌtɪv dɛnsɪtiː/
ratio of the density (mass per volume) of a substance to the density of a given reference material