Used in a very general sense, effect size is the quantitative measure of strength in a statistical relation (e.g. correlations, differences of means, etc.) and it is defined as such in Wikipedia.
In the literature, the wording "effect size" is mostly used in meta studies, where it refers to various techniques that allow combining multiple effect-sizes. 
- Practical Meta-Analysis Effect Size Calculator. This is an online tool developed by David B. Wilson, Ph.D., George Mason University
- Brand A, Bradley MT, Best LA, Stoica G (2011). Multiple trials may yield exaggerated effect size estimates, The Journal of General Psychology, 138(1), 1–11 doi=10.1080/00221309.2010.520360 PDF reprint
- Ellis, Paul D. (2010). The Essential Guide to Effect Sizes: An Introduction to Statistical Power, Meta-Analysis and the Interpretation of Research Results. United Kingdom: Cambridge University Press.
- Kelley, Ken; Preacher, Kristopher J. (2012). "On Effect Size". Psychological Methods 17 (2): 137–152. doi:10.1037/a0028086.