Here's how it's defined:

* Density (ρ) = Mass (m) / Volume (V)

Density is a measure of how much mass is contained in a given volume. It's a fundamental property of matter and is often used in calculations involving:

* Fluid dynamics: Understanding the behavior of liquids and gases

* Gravitation: Calculating gravitational forces

* Materials science: Characterizing the properties of different materials

Example:

Let's say you have a block of wood with a mass of 10 kg and a volume of 20 cubic meters. The density of the wood would be:

* ρ = 10 kg / 20 m³ = 0.5 kg/m³

Therefore, the density of the wood is 0.5 kilograms per cubic meter.

Note: RHO is also sometimes used in other contexts in physics, but density is its most common meaning.