specific gravity
noun [ U ] physics specialized uk/spəˌsɪf.ɪk ˈɡræv.ɪ.ti/ us/spəˌsɪf.ɪk ˈɡræv.ə.t̬i/
比重，相对密度（现更常用 relative density）
the mass of a particular volume of a substance when compared with the mass of an equal volume of water at 4°C. A more modern name for this is relative density.