A Fermi gas is a collection of particles that follow the principles of quantum mechanics, specifically the Pauli exclusion principle. This principle states that no two identical fermions, such as electrons or neutrons, 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 neutron stars. In these systems, the interactions between particles can lead to unique phenomena, such as superconductivity and the stability of matter under extreme conditions.