Optically identified BL Lacertae objects from the Sloan Digital Sky Survey

We present a sample of 347 BL Lacertae (BL Lac) candidates identified from 2600 deg^2 of the Sloan Digital Sky Survey (SDSS) spectroscopic database. The candidates are primarily selected to have quasi-featureless optical spectra and low proper motions as measured from SDSS and USNO-B positions. High proper motion quasi-featureless objects (QFOs), mostly DC white dwarfs, populate a well defined region of color space, approximately corresponding to blackbodies with temperatures in the range 7000--12000 K. QFOs with X-ray and/or radio counterparts loosely follow a track in color space that corresponds to power-law continua plus host galaxy starlight, with typical power-law slopes in the range 1<\alpha_{opt}<2 (f_{\nu}\propto \nu^{-\alpha}). Based largely on the relatively clean color segregation between high proper motion objects and objects with other evidence of a BL Lac nature, in the form of X-ray/radio counterparts or measurable redshifts, we subdivide the sample into 236 probable BL Lac candidates and 111 additional less probable candidates. The probable BL Lac candidates have multi-wavelength properties consistent with the range of previously known BL Lacs, with an apparent preponderance of objects with synchrotron peaks at relatively high energies (HBL/XBL-type). The majority of the 154 objects with measurable redshifts have z