The Greek letter rho (ρ) is used to denote density because it is the first letter of the Greek word "pyknos," which means "dense" or "thick." Density is a measure of how much mass is packed into a given volume, so it makes sense to use a letter from the word "dense" to represent it.

In addition, rho is a relatively easy letter to write and remember, which makes it a convenient choice for scientific notation. It is also distinct from other Greek letters that are used to represent other physical quantities, such as alpha (α) for angle, beta (β) for velocity, and gamma (γ) for energy.

Therefore, rho is the most commonly used symbol for density in physics, chemistry, and other scientific disciplines.