  Observations of the quiescent X-ray nova XTE J1118+480 with the new 6.5-m MMT
have revealed that the velocity amplitude of the dwarf secondary is 698 +/- 14
km/s and the orbital period of the system is 0.17013 +/- 0.00010 d. The implied
value of the mass function, f(M) = 6.00 +/- 0.36 solar masses, provides a hard
lower limit on the mass of the compact primary that greatly exceeds the maximum
allowed mass of a neutron star. Thus we conclude that the compact primary is a
black hole. Among the eleven dynamically established black-hole X-ray novae,
the large mass function of XTE J1118+480 is rivaled only by that of V404 Cyg.
We estimate that the secondary supplies 34% +/- 8% of the total light at 5900A
and that its spectral type is in the range K5V to M1V. A double-humped I-band
light curve is probably due to ellipsoidal modulation, although this
interpretation is not entirely secure because of an unusual 12-minute offset
between the spectroscopic and photometric ephemerides. Assuming that the light
curve is ellipsoidal, we present a provisional analysis which indicates that
the inclination of the system is high and the mass of the black hole is
correspondingly modest. The broad Balmer emission lines (FWHM = 2300-2900 km/s)
also suggest a high inclination. For the range of spectral types given above,
we estimate a distance of 1.8 +/- 0.6 kpc.
