Baer, Wolf, and Risley's 1968 article  is the standard description of Applied Behavior Analysis (ABA). ABA is the "process of applying sometimes tentative principles of behavior to the improvement of specific behaviors, and simultaneously evaluating whether or not any changes noted are indeed attributable to the process of application -- and if so, to what parts of that process." In short, ABA is a "self-examining, self-evaluating, discovery-oriented research procedure for studying behavior" (p. 91).
Baer, D.M., Wolf, M.M., Risley, T.R. (1968). "Some current dimensions of applied behavior analysis". Journal of Applied Behavior Analysis 1 (1): 91–97. doi:10.1901/jaba.1968.1-91 http://www.pubmedcentral.nih.gov/articlerender.fcgi?pubmedid=16795165