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 \sim 13 and 9 months , the absorption lines in the kinematic component with highest outflow velocity exhibited mean redward velocity shifts of \sim 35 and 55 km s ^ { -1 } , respectively .
The rate of velocity decrease was 2.2 \pm 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 .