The ground state is the lowest-energy state of a quantum system.
A quantum system can occupy discrete energy levels determined by its Hamiltonian, the operator representing its total energy. The state with the smallest allowed energy is called the ground state. Any state with greater energy is an excited state. For an atom, the ground state generally places its electrons in the lowest available configuration permitted by quantum rules.
The ground state does not necessarily have zero energy. Confinement and the uncertainty principle can leave a system with nonzero zero-point energy, even at absolute zero. Absolute zero means thermal energy is minimized; it does not mean every quantum system becomes motionless or loses all energy.
Ground states are central to spectroscopy, chemistry, condensed-matter physics, and quantum computing. A system may relax toward its ground state by emitting photons or other excitations, although selection rules can make some transitions slow or forbidden. The term should not be confused with the vacuum state, which refers specifically to the lowest-energy state of a quantum field.