The evidence summarized in the preceding sections paints the following picture for the physical nature of LINERs. For reasons yet to be fully understood, the narrow emission-line regions of AGNs evidently can allow a wide range of ionization parameters (the ratio of the density of ionizing photons to the density of gas at the illuminated face of a cloud; U Lion / 4 cnr2). Those with U 10-2-10-1 are conventionally denoted ``Seyferts;'' low-ionization objects with U 10-3.5-10-2.5 are called ``LINERs'' (Halpern and Steiner 1983; Ferland and Netzer 1983; Ho et al. 1993). There is no sharp division, other than that imposed for sheer taxonomical convenience, between the two groups. Like Seyferts, some LINERs (~ 20%) come equipped with a BLR. The others either do not have BLRs, or their BLRs are obscured from the observer. At least some of the type 2 LINERs definitely have hidden BLRs that can only be seen in scattered light.
Most LINERs inhabit large, bulge-dominated galaxies, the very galaxies that evidently are most prone to host supermassive black holes. Thus, LINERs can be identified with the quiescent black-hole remnants from the quasar era. In the present epoch, the supply of gas available for powering the central engines is much curtailed, and the resulting low accretion rates lead to an advection-dominated mode of accretion that manifests itself in the low luminosity output and in the peculiar SEDs of the nuclei. The diminished nuclear power naturally accounts for the low values of U in LINERs, and the likely preponderance of ADAFs in nearby galactic nuclei may explain why LINERs are so ubiquitous. Two additional effects may help to further reduce the ionization state of the line-emitting gas. The modification of the SED from UV to X-ray energies, most noticeable by the absence of the UV excess, leads to an overall hardening of the ionizing spectrum. The exceptional strength of the radio component, on the other hand, increases the effectiveness of cosmic-ray heating of the gas by the energetic electrons (e.g., Ferland and Mushotzky 1984). Both effects result in an enhancement of the low-ionization lines.