A Fermi gas is a collection of particles that follow the principles of Fermi-Dirac statistics. These particles, known as fermions, include electrons, protons, and neutrons, which obey the Pauli exclusion principle, meaning no two fermions can occupy the same quantum state simultaneously. This behavior is crucial in understanding the properties of materials, especially at very low temperatures. In a Fermi gas, the particles fill available energy states up to a maximum energy level called the Fermi energy. At absolute zero, all states below this energy are filled, while those above remain empty. This concept is essential in fields like condensed matter physics and astrophysics, particularly in the study of degenerate matter found in white dwarfs and neutron stars.