Specific gravity

Specific Gravity (sg) is the ratio of the density of a liquid against the density of pure water.

Typically used in the measurement of salt content in brackish, marine or reef aquarium. A hydrometer, or a refractometer are typical devices used to measure sg.

Although sg is a good way of measuring salt content, it can be influenced by other factors like dissolved organic matter in planted aquaria as well as any medications used.