The extent to which information about a program, project, or instructional material collected in one setting can be used to reach a valid judgment about how it will perform in other settings.
Yarbrough, D. B., Shulha, L. M., Hopson, R. K., and Caruthers, F. A. (2011). The program evaluation standards: A guide for evaluators and evaluation users (3rd ed.). Thousand Oaks, CA: Sage.