Chemical Potential in thermodynamics is defined by $$\mathrm{d}U= T\mathrm{d}S - P\mathrm{d}V + \sum_{i=1}^n \mu_i \,\mathrm{d}N_i$$ where $U$ is the Internal Energy of the system; $S$ is the Entropy of the system; $N$ is the Number of Particles in the system; $V$ is the Volume of the system; $P$ is the systems' Pressure; $T$ is the absolute (thermodynamic) temperature of the system and $\mu$ is the Chemical Potential of the system.