An "Grand Canonical Partition Function" for a grand canonical ensemble, a system that can exchange both heat and particles with the environment, which has a constant temperature and a chemical potential.
$\Xi = \sum_{N_A, N_B, ...} Z(N_A, N_B, ...) \cdot \lambda_A^{N_A} \cdot \lambda_B^{N_B} \cdot ...$, where $Z(N_A, N_B, ...)$ is the canonical partition function for the given number of particles $A, B, ...,$, and $\lambda_A, \lambda_B, ...$ are the absolute activities of particles $A, B, ...$.