  We have created a variety of composite quasar spectra using a homogeneous
data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The
quasar sample spans a redshift range of 0.044 <= z <= 4.789, and an absolute r'
magnitude range of -18.0 to -26.5. The median composite covers a rest
wavelength range from 800 - 8555A, and reaches a peak signal-to-noise ratio of
over 300 per 1A resolution element in the rest frame. We have identified over
80 emission line features in the spectrum. Emission line peak velocity shifts
of the broad permitted and semi-forbidden lines are strongly correlated with
ionization energy, as previously suggested, but we find that the narrow
forbidden lines are also shifted by amounts which are strongly correlated with
ionization energy. The magnitude of the forbidden line shifts is < 100 km/s,
compared to shifts of up to 550 km/s for some of the permitted and
semi-forbidden lines. At wavelengths longer than the Ly-a emission, the
continuum of the geometric mean composite is well-fit by two power-laws, with a
break at approximately 5000A. The frequency power law index, alpha_nu, is -0.44
from 1300 - 5000A, and -2.45 redward of about 5000A. The abrupt change in slope
can be accounted for partly by host galaxy contamination at low redshift.
Stellar absorption lines, including higher-order Balmer lines, seen in the
composites suggest that young or intermediate age stars make a significant
contribution to the light of the host galaxies. An electronic table of the
median quasar template is available.
