A LINER (low-ionization nuclear emission-line region) is a type of gaseous region common in the centers of many kinds of galaxies. Some of these have been shown to be low-luminosity active galactic nuclei, perhaps an extension of Seyfert activity to the lowest levels, implying that the whole phenomenon of nuclear activity occurs in a significant fraction of bright galaxies. A LINER's spectrum is dominated by emission lines from low ionization states, such as OII, NII, and SII, with only weak lines from highly ionized states.