In chemistry and physics, the ground state is defined as the lowest allowed energy state of an atom, molecule, or ion. In other words, the ground state represents the most stable configuration. If there is more than one possible ground state, degenerate states are said to exist. Even though the species may possess some level of energy, the ground state is considered to have zero-point energy in comparison to other states. If a species has energy greater than the ground state, it is said to be in an excited state.
Electrons provide a good example of ground and excited states. If an electron absorbs energy, it may jump to an excited state. At some point, the electron will return to the ground state, typically giving off a photon in the process.