Anxiety is a descriptive term which refers to the changes in a performance produced by an aversive or preaversive stimulus. These changes include a decrease in the frequency of many operant performances which might otherwise have occurred under these circumstances if the preaversive stimulus had not occurred. They also include an increase in the frequency of performances which in the past have terminated or reduced the magnitude of the aversive stimulus. The term refers to changes in the frequency of broad classes of behaviours in the individual's repertoire. Because many performances are altered, we speak of anxiety as a state of the organism.