Effect size refers to a statistical measure of the strength and magnitude of the relationship between two variables in a research study. It is used to quantify the size of the difference or the strength of the relationship between two groups or variables, beyond just indicating whether a relationship exists. Effect sizes can help researchers interpret the practical significance of their findings and compare results across different studies. Common effect size measures include Cohen's d, eta-squared, and r-squared. A larger effect size indicates a stronger relationship between variables, while a smaller effect size suggests a weaker relationship.