SpGr, short for Specific Gravity, is a term commonly used in science and engineering to refer to the density of a substance relative to the density of water. However, there are various synonyms that are also used interchangeably to describe SpGr. Some of the common synonyms include relative density, specific weight, specific gravity ratio, and specific volume. These terms essentially provide the same meaning as SpGr, but may be used in different contexts or situations. Regardless of the terms used, they all refer to the ratio of the density of a substance to that of another substance, usually water.