When a sense organ reacts less and less to a persistent or repetitive stimulus. For example, after a while, despite a smell persisting, a person's nose may no longer signal its presence to the brain so he or she does not sense it any more; another example is that we are normally unaware of the pressure of clothes on our skin as pressure-sensitive nerve endings adapt; regular background noise may go unnoticed because it has been screened out by the cochlear nerve that links ear and brain.