Definition
Motivating Operation (MO): Refers to an environmental variable that momentarily alters the effectiveness of some stimulus, object, or event as a reinforcer or punisher. Additionally, it can momentarily alter the current frequency of all behavior that has been reinforced or punished by that stimulus, object, or event in the past.
Example
For example, deprivation of food increases the effectiveness of food as a reinforcer (making the subject more likely to engage in behaviors that lead to food). Conversely, satiation of food decreases the effectiveness of food as a reinforcer.