We discuss the physical conditions and excitation of the nucleus of M81 by combining ground-based optical spectra, archival Hubble Space Telescope (HST) ultraviolet (UV) and optical spectral, and published X-ray measurements. This well-known low-ionization nuclear emission-line region (LINER) is representative of a general class of objects thought to be the less luminous counterparts of active galactic nuclei (AGNs) present in Seyfert galaxies and quasars. A featureless UV continuum, most likely of nonstellar origin, is detected unambiguously for the first time in M81. Compared with ''classical,'' more luminous AGNs, the UV continuum is weak relative to the X-rays, the slope between 1200 and 3000 Angstrom is significantly steeper (f(nu) proportional to nu(-2.0+/-0.3) rather than nu(-0.5) to nu(-1)), and the ''big blue bump'' is absent. The nonstellar continuum of the nucleus has an absolute blue magnitude of -11.6, only a factor of 5 brighter than that of the least luminous Seyfert 1 nucleus in NGC 4395. We suggest that these characteristics of the spectral energy distribution may be a generic property of galactic nuclei having low-level activity, perhaps being a manifestation of extremely low accretion rates. The UV-optical spectrum exhibits broad emission lines with FWHM approximate to 2500 km s(-1), further reinforcing the similarity to Seyfert 1 nuclei. Detailed properties have been derived fdr the regions emitting narrow and broad lines. The narrow-line region is characterized by velocities of 200-1000 km s(-1) a large, radially decreasing gradient in density ranging from a few times 10(2) to 10(7) cm(-3), and a spatial extent of less than I to several parsecs from the central ionizing source. We deduce that the gas has roughly solar metallicity, an ionization parameter of similar to 10(-3.3), and electron temperatures typical of photoionized nebulae. Lacking information on the structure of the broad-line region, we can obtain only crude estimates of its density (10(9)-10(10) cm(-3)), ionization parameter (less than or similar to 10(-2.8), and radius (0.001-0.004 pc). Combining the velocities and dimensions of the broad-line region, and assuming that gravity dominates the motions of the clouds, we derive a central mass of (0.7-3) x 10(6) M(.) for the nucleus of M81. The low bolometric luminosity of the nucleus implies that it is radiating only at (2-10) x 10(-4) of the Eddington limit. Our extensive ground-based monitoring reveals that the broad component of the H alpha emission line has not varied significantly during the past 15 yr, although X-ray variability on timescales of years has been reported. This variability pattern is not yet understood. The broad H alpha luminosity (1.8 x 10(39) ergs s(-1)) is 15 times larger than in NGC 4395, assuming that the reddening intrinsic to the broad-line region is negligible in both objects.
