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In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any configuration of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum).
The lifetime of a system in an excited state is usually short: spontaneous or induced emission of a quantum of energy (such as a photon or a phonon) usually occurs shortly after the system is promoted to the excited state, returning the system to a state with lower energy (a less excited state or the ground state).
Example: the hydrogen atom
A simple example of this concept comes by considering the hydrogen atom.
The ground state of the hydrogen atom corresponds to having the atom's single electron in the lowest possible orbit (that is, the spherically symmetric "1s" state, which has the lowest possible quantum numbers). By giving the atom additional energy (for example, by the absorbsion of a photon of an appropriate energy), the electron is able to move into an excited state (one with one or more quantum numbers greater than the minimum possible). If the photon has too much energy, the electron will cease to be bound to the atom, and the atom will become ionised.
Once the electron is in its excited state, we deem the hydrogen atom to be in its excited state. The atom may return to a lower excited state, or the ground state, by emitting a photon with a characteristic energy. Emission of photons from atoms in various excited states leads to a spectrum showing a series of characteristic emission lines (including, in the case of the hydrogen atom, the Lyman series, the Balmer series, the Paschen series, the Brackett series.)
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