An emission or absorption line in a spectrum that arises when an electron moves between two energy levels in an atom. A jump to a higher level requires an input of energy, and produces a dark absorption line. A drop to a lower level releases energy, producing a bright emission line. At visible wavelengths, spectral lines from elements such as iron are largely due to jumps involving the outer electrons, but X-rays are produced from the inner electron shells. The precise wavelength (or energy) is a hallmark of the atom involved in producing the line.