When your attention is drawn to a stimulus, but you aren't looking at (or otherwise focusing on) it.
This is interesting to psychologists and neurologists because it helps explain how new stimulus is brought to our attention. Areas in our temporal lobe alert us to stimulus in our peripheral vision, triggering us to move our eyes or turn our heads. Once we have the stimulus directly in our focus, we are using overt orienting.
This phrase is primarily used in animal psychology and the psychology of attention.