An energetically higher solution to the wavefunction of a particular system. An electron in the ground state can be excited by a photon of light, or in some cases, thermal or chemical energy, driving it into a higher molecular orbital. It can remain in that state until it is depleted, either by internal vibrational relaxation, by collision with other molecules that cause orbital mixing, or by radiative decay (fluorescence). The physical and chemical nature of a molecule in the excited state is often very different than that of the ground state.