A Fermi gas is a collection of particles that follow the principles of quantum mechanics and obey the Pauli exclusion principle. This principle states that no two identical fermions, such as electrons or protons, can occupy the same quantum state simultaneously. As a result, fermions fill available energy levels up to a maximum energy, known as the Fermi energy. Fermi gases are important in understanding the behavior of electrons in metals and the properties of degenerate matter found in astrophysical objects like white dwarfs and neutron stars. In these systems, the effects of quantum statistics become significant, leading to unique physical properties that differ from classical gases.