The Complete Sample of 1 Jansky BL Lacertae Objects. I. Summary Properties

This paper describes the first homogeneous, flux-limited sample of radio-selected BL Lac objects, taken from the 1 Jy survey of Kuhr et al. These 34 BL Lac objects comprise the only well-defined sample of BL Lac objects outside the X-ray band. The selection criteria include flat radio spectra, optical counterparts visible on Sky Survey Plates, and optical emission lines weaker than 5 A equivalent width in the rest frame. With two exceptions, all of the BL Lac objects also have high optical polarization, P_opt_ > 3%. Redshifts from weak emission lines, usually narrow [O II] λ3727, [O III] λ5007, or broad Mg II λ2798, are available for ~ 3/4 of the sample. The nearby (z < 0.2) BL Lac objects, at least, sit in elliptical galaxies and may occur in groups or small clusters of galaxies. Gravitational lensing appears to affect one of the high-redshift (z >~ 0.5) BL Lac objects and could be important in others as well. The radio number counts derived from the 1 Jy BL Lac sample are roughly Euclidean over the range between 1 and ~ 5 Jy, and there is evidence for evolution at the 2 σ level. The local radio luminosity function of BL Lac objects is derived and the total number density for P_5 GHz_ >~ 6 x 10^31^ ergs s^-1^ Hz^-1^ is ~ 40 Gpc^-3^.