an atom that has electrons in higher energy levels than normal; an atom that has absorbed energy, usually in the form of a photon. (see ground state)

an atom which possesses more energy than a normal atom of that species

An atom which has lost one of its inner electrons (an ion) and has a higher energy value. The extra energy is that needed to remove the electron from the nucleus, which seeks to return to its ground state by rearranging its electrons and emitting the excess energy as an X-ray quantum.