What is Applied Behavior Analysis?

The scientific approach for discovering environmental variables that reliably influence socially significant behavior and for developing a technology of behavior change that takes practical advantage of those discoveries. - Cooper, Heron, and Heward

Human behavior is the organism's interaction with the environment. It is literally everything people do that creates a measurable change to at least one aspect of the environment.

The systematic approach for seeking and organizing knowledge about human behavior. 

Traditionally, both the layperson and the psychologist have tried to understand behavior by seeing it as the outcome of what we think, what we feel, what we want, what we, calculate, etc. But we don't have to think about behavior that way. We could look upon it as a process that occurs in its own right and has its own causes. And those causes are, very often found in the external environment."

The environment refers to the conglomerate of real circumstances in which the organism exists.