What characteristic of a fluid does the term "specific gravity" describe?

Specific gravity is a measure that describes the density of a fluid in relation to the density of a reference substance, typically water. Essentially, it allows one to understand how heavy or light a fluid is compared to water, which has a specific gravity of 1. If a fluid has a specific gravity greater than 1, it is denser than water; if it is less than 1, it is less dense than water.

This concept is crucial in hydraulics and fluid mechanics, as density plays a significant role in determining behaviors such as buoyancy, pressure exertions, and the energy required for fluid movement. The other options refer to different properties: viscosity pertains to a fluid's resistance to flow, flow rate refers to the volume of fluid passing a point over time, and pressure is the force exerted by a fluid per unit area. Each of these is fundamentally different from specific gravity, which specifically focuses on the relationship of a fluid’s density to that of water.