  We report the discovery of an ultrafaint dwarf (UFD) galaxy, Pegasus W,
located on the far side of the Milky Way-M31 system and outside the virial
radius of M31. The distance to the galaxy is 915 (+60/-91) kpc, measured using
the luminosity of horizontal branch (HB) stars identified in Hubble Space
Telescope optical imaging. The galaxy has a half-light radius (r_h) of 100
(+11/-13) pc, M_V = -7.20 (+0.17/-0.16) mag, and a present-day stellar mass of
6.5 (+1.1/-1.4) x 10^4 Msun. We identify sources in the color-magnitude diagram
(CMD) that may be younger than ~500 Myr suggesting late-time star formation in
the UFD galaxy, although further study is needed to confirm these are bona fide
young stars in the galaxy. Based on fitting the CMD with stellar evolution
libraries, Pegasus W shows an extended star formation history (SFH). Using the
tau_90 metric (defined as the timescale by which the galaxy formed 90% of its
stellar mass), the galaxy was quenched only 7.4 (+2.2/-2.6) Gyr ago, which is
similar to the quenching timescale of a number of UFD satellites of M31 but
significantly more recent than the UFD satellites of the Milky Way. Such
late-time quenching is inconsistent with the more rapid timescale expected by
reionization and suggests that, while not currently a satellite of M31, Pegasus
W was nonetheless slowly quenched by environmental processes.
