  We report an intrinsic absorber with decreasing outflow velocity in the
Seyfert 1 galaxy NGC 3783. This is the first detection of a change in radial
velocity in an outflow associated with a Seyfert galaxy. These results are
based on measurements from 18 observations with the Space Telescope Imaging
Spectrograph aboard the Hubble Space Telescope, obtained between 2000 February
and 2002 January. In two intervals separated by ~13 and 9 months, the
absorption lines in the kinematic component with highest outflow velocity
exhibited mean redward velocity shifts of ~35 and 55 km/s, respectively. The
rate of velocity decrease was 2.2 +/- 0.6 times more rapid in the second
interval. No variations in absorption velocities were detected in the other
kinematic components. We explore potential interpretations of the observed
velocity shifts: radial deceleration of the UV absorber due to a change in
either the speed or direction of motion of the outflow, and the evolution of a
continuous flow across our line of sight to the emission source.
