The term scan speed is generally cited at an angle per second, e.g., 30"/s, which I take to mean the rate at which the aim moves to a new portion of the celestial sphere. It might be used for the rate at which it moves in relation to the ground underneath, or that may be virtually the same, the difference being 15"/s (by my calculation) for a target on the ecliptic. The concept is relevant to surveys (e.g., slew surveys) and observations of extended targets in both cases where the telescope's aim is being moved during the observation. There is likely a tradeoff between the scan speed used and the quality of the observation.
Some space telescopes and probes are designed to observe while spinning, with a fixed scan speed.