Final answer:
The relationship between exercise and the reduction of mental illness symptoms is described as a correlation, as exercise is associated with changes in mental health but does not necessarily cause these changes directly.
Step-by-step explanation:
The relationship between the reduction of mental illness symptoms and exercise is best described as correlation. A correlation indicates that there is an association between two variables -- in this case, exercise and mental health -- such that changes in one are associated with changes in the other. However, establishing a correlation does not necessarily imply a causation, which would mean that one variable directly causes the other to change. This is because there could be other factors or confounding variables at play. So while exercise is often correlated with improved mental health, it is not accurate to say with certainty that exercise directly causes the reduction of mental illness symptoms without rigorous, controlled experiments to rule out other variables.