APA Dictionary of Psychology

the effect on the current performance of a research participant of the experimental conditions that preceded the current conditions; where such an effect is significant, it may be difficult to determine the specific influence of the variable under study. For example, in a crossover design in which a particular drug is administered to nonhuman animals in the first experimental condition, a carryover effect would be evident if the drug continued to exert an influence on the animals’ performance during a subsequent experimental condition. Also called holdover effect. See wash-out period.