A slew survey is a survey making use of a telescope's movement, e.g., during the time it is being moved to point to another object. operating an imager while the telescope is moving, and (possibly later) processing the data to identify the coordinates of observed sources and the timing of the observations. They are carried out using telescopes that can snap useful images during the movement. The term slew survey is used for surveys making use of the otherwise-unused time during movements of the telescope, but is also probably used for surveys where the telescope movement involved is purely for the benefit of the survey, "surveying on the move" to efficiently cover the survey field.
Slew is a term for the modification of the attitude of a spacecraft and probably is used for re-pointing ground telescopes as well.
Naturally, locating the sources requires knowledge of the attitude of the telescope at all moments while it is being repointed/moved. Such an observation is a slew observation, as opposed to a pointed observation, i.e., one consisting of aiming the instrument at specific coordinates. Slew surveys, by nature, are shallow, detecting only sources sufficiently strong to trigger some detector activity during the moments when the imager is on the coordinate.
The technique of using "slew time" is used for space telescopes, particularly X-ray telescopes, that point by changing the attitude of the satellite. Given the limited lifetime and fuel of space missions, it is means to accomplish additional investigation. All-sky surveys can be carried out over a course of months even while the telescope is "constantly" being used for pointed observations.