The Greek letter mu, written μ, commonly denotes chemical potential in thermodynamics.
Chemical potential measures how the Gibbs free energy changes when the amount of a component changes, with temperature, pressure, and the amounts of other components held appropriately fixed. For component i, it is often written μᵢ = (∂G/∂nᵢ) at constant temperature, pressure, and other component amounts. Its units are joules per mole in molar thermodynamics.
The symbol μ appears naturally in the fundamental thermodynamic relation dG = VdP − SdT + Σμᵢdnᵢ. Chemical potential helps predict diffusion, phase equilibrium, osmosis, and the direction of chemical reactions. At equilibrium between phases, the chemical potential of each component is equal across the phases.
Mu is easy to confuse with other symbols because it has many uses. In physics, μ can represent permeability, dynamic viscosity, reduced mass, or the micro- prefix. The symbol is not a definition by itself: notation can vary between authors, but lowercase μ is the conventional thermodynamic choice for chemical potential.