A method in cognitive psychology, backward masking occurs when a visual stimulus is presented to a subject which is immediately followed by another visual stimulus which 'masks' the effect of the first stimulus. It is 'masking' the effects of one stimulus by presenting another immediately after it.
For example, researchers may flash a series of numbers on a screen and milliseconds later follow it up with a series of shapes and lines. Because the second image is 'masking' the first the subject will not be able to recall the first stimulus--the series of numbers. Backward masking is used in techniques such as subliminal messaging.