Basic Definition:
Frame averaging occurs when multiple frames are acquired with the detector and the sample in the same position relative to one another, and then averaged together into a composite image. Frame averaging can improve the signal-to-noise ratio by reducing the contribution of random noise. However, it can create image blurring if the sample shifts over the course of the scan.
More Information:
While increased exposure time can improve signal to noise ratio up to a point, it will eventually result in detector saturation. If all other factors are kept constant, frame averaging can theoretically achieve the same SNR improvement as increased exposure time, but without the risk of detector saturation.