 One of the more challenging words to define, ironically, is behavior. Behavior specifically is the activity of living organisms. I actually like that definition the best. However, we can get very technical here. One of those would be the portion of an organism's interaction with its environment that is characterized by detectable displacement in space through time of some part of the organism and the results in the measurable change in at least one aspect of the environment. The reason I don't really like that is because it leaves off things such as private behavior or, as Skinner would have said, covert behavior. Thinking. A simple way to think about behavior is if it ends in ING, it's probably a behavior.