In research, the ecological validity of a study means that the methods, materials and setting of the study must approximate the real-world that is being examined. Unlike internal and external validity, ecological validity is not necessary to the overall validity of a study.[not specific enough to verify]
External validity versus ecological validity 
Ecological validity is often confused with external validity (which deals with the ability of a study's results to generalize). While these forms of validity are closely related, they are independent—a study may possess external validity but not ecological validity, and vice versa. For example, mock-jury research is designed to study how people might act if they were jurors during a trial, but many mock-jury studies simply provide written transcripts or summaries of trials, and do so in classroom or office settings. Such experiments do not approximate the actual look, feel and procedure of a real courtroom trial, and therefore lack ecological validity. However, the more important concern is that of external validity—if the results from such mock-jury studies generalize to real trials, then the research is valid as a whole, despite its ecological shortcomings. Nonetheless, improving the ecological validity of an experiment typically improves the external validity as well.
In the original meaning of this term however it would be impossible to refer to a study's 'ecological validity' since this can only be a property of stimuli used in a perceptual experiment. Brunswick first introduced this term but the popular use (meaning basically 'realistic') has overtaken it, at least in A Level Psychology circles. For the original (and some feel correct) use see the entry on 'ecological validity - perception' and especially the paper by Hammond, 1998, which is referred to there. This can be accessed at: http://www.albany.edu/cpr/brunswik/notes/essay2.html . Brunwick's term which is closer to the current 'pop' meaning of ecological validity is 'representative design'.
See also 
- Brewer, M. (2000). Research Design and Issues of Validity. In Reis, H. and Judd, C. (eds) Handbook of Research Methods in Social and Personality Psychology. Cambridge:Cambridge University Press.
- Shadish, W., Cook, T., and Campbell, D. (2002). Experimental and Quasi-Experimental Designs for Generalized Causal Inference Boston:Houghton Mifflin.