An Etymological Dictionary of Astronomy and Astrophysics

A type of galactic nucleus that is defined by its spectral line emission. The lines are very weak, the most prominent ones being from low ionization states (such as [O II], [N II], [S II] and [OI]). There is so far no generally accepted interpretation of the spectra of
liners. It is likely that galaxies of different histories may have their nuclei with liner-type spectra. → retired galaxy.

See also: Short for → Low-Ionization Nuclear Emission-line Region. The term liner was first introduced by T. M. Heckman (1980, A&A 87, 152).