Final answer:
The term that describes the spread of treatment from the experimental group to the control group is 'treatment diffusion,' which undermines the differences that should be maintained in an experiment.
Step-by-step explanation:
The term used to describe the spread of the treatment from the experimental group to the control group, which tends to reduce the difference between the two conditions, is treatment diffusion. When conducting an experiment, it is crucial for the control group to serve as a basis for comparison and to control for chance factors that might influence the results of the study. The experimental manipulation should be the only difference between the groups. However, if treatment diffusion occurs, this can compromise the validity of the study's findings, as the control group will inadvertently receive some or all aspects of the experimental treatment.