Definitions for "Effect size"
The difference between two population means divided by the standard deviation of either population.
The size of the difference or effect being studied, such as the percentage achieving a goal or a mean number.
The difference in measured outcomes attributed to a therapeutic intervention. This term is encountered in meta-analyses when different studies have measured different things. For example, results of an asthma study which measured FEV1 could be combined with those of another study which measured return visits to the ED using a statistically derived generic effect size. Do you prefer skiing or red wine