Behavior is any action an organism uses to adjust to the environment. But, there has always been divergent opinions about what ought to be included under the category of behavior. In the narrower sense, only behaviors or actions that can be sensed or are visible are categorized as behaviors. Behaviorists and psychologists argue that change in the environment are only seen when the behaviors are observable, which are also called overt behaviors.
Human actions however, are not limited to observable actions, and there are wide ranges of emotions, thought processes which are not seen or sensed.
In broader sense, behavior is psycho-physical in origin and includes anything that the individual does or experiences. For instance, ideas, dreams, glandular responses, running, reading in silent etc are actions that are not overtly observed, yet, they hold significant value influencing the overt behaviors and bringing change to the environment. These behaviors are called covert behaviors.
Here are the differences between overt and covert behavior.