$\rho$ — Rho

Lowercase rho (ρ) most often denotes density — mass per unit volume. In electricity it is resistivity, the material property behind resistance, and in quantum mechanics it can denote charge density.

SI unit: kg/m³ (as density)

Used in

• Density ρ = m/V
• Resistivity in R = ρL/A
• Charge density in electromagnetism