Emission lines in the spectrum of the Sun’s corona caused by highly ionized atoms. Only a small number of coronal lines are emitted at visible wavelengths, and these can be studied from the ground only during eclipses or with coronagraphs. The visible coronal lines are forbidden lines (caused by transitions of very low probability in highly ionized atoms). Examples include the so-called red and green coronal lines at wavelengths of 637.5 and 530.3 nm due to Fe⁹⁺ (iron lacking 9 electrons) and Fe¹³⁺ (iron lacking 13 electrons), respectively, which were once thought to be caused by an unknown element, dubbed ‘coronium’. Many more coronal lines are found at ultraviolet and X-ray wavelengths and can be detected only from space. These ultraviolet and X-ray lines have been observed not only in the Sun but also in a wide range of other stars with the Extreme Ultraviolet Explorer, the Chandra X-Ray Observatory, and XMM-Newton.