In quantum mechanics, a stationary state of higher energy than the lowest stationary state or ground state of a particle or a system of particles. Customarily, only bound stationary states, which generally are at most denumerably infinite in number, are spoken of as excited, although the formal quantum theory often treats the noncountable unbound stationary states on an equal footing with the bound states. Conventionally, the excited states are ranked in order of increasing energy; that is, the second excited state has higher energy than the first, which lies higher than the zeroth or ground state.