The partition function is a central concept in statistical mechanics that provides a link between the microscopic properties of molecules and the macroscopic observable properties of a system. It is denoted as Z and is a sum over all possible states of a system, weighted by the Boltzmann factor, \( e^{-E_i/k_BT} \), where \( E_i \) is the energy of state i, \( k_B \) is the Boltzmann constant, and T is the temperature.