Definition reproduced by permission from Amanda N. Kelly at Behaviorbabe.
The removal of a stimulus upon the occurrence of a behavior that is likely to lead to that behavior happening again in the future. For example, a loud fire alarm prompts you to exit a building. If it is the absence of being caught in a burning building, being reprimanded or potentially fined for not exiting the building or even the removal of the loud, blaring noise of the alarm are enough to suggest that when you hear an alarm again in the future, you are likely to continue to engage in the behavior (i.e. exit the building).
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