Behavior or behaviour (see spelling differences) refers to the actions or reactions of an object or organism, usually in relation to the environment. Behavior can be conscious or unconscious, overt or covert, and voluntary or involuntary.

In animals

In animals, behavior is controlled by the endocrine system and the nervous system. The complexity of the behavior of an organism is related to the complexity of its nervous system. Generally, organisms with complex nervous systems have a greater capacity to learn new responses and thus adjust their behavior.

In psychology

Human behavior (and that of other organisms and mechanisms) can be common, unusual, acceptable, or unacceptable. Humans evaluate the acceptability of behavior using social norms and regulate behavior by means of social control. In sociology, behavior is considered as having no meaning, being not directed at other people and thus is the most basic human action. Animal behavior is studied in comparative psychology, ethology, behavioral ecology and sociobiology.

Behavior became an important construct in early 20th century Psychology with the advent of the paradigm known subsequently as "behaviorism." Behaviorism was a reaction against so-called "faculty" psychology which purported to see into or understand the mind without the benefit of scientific testing. Behaviorism insisted on working only with what can be seen or manipulated and in the early views of John B. Watson, a founder of the field, nothing was inferred as to the nature of the entity that produced the behavior. Subsequent modifications of Watson's perspective and that of so-called "classical conditioning" (see under Ivan Pavlov) led to the rise of Operant Conditioning, a theory advocated by B.F. Skinner, which took over the academic establishment up through the 1950s and was synonymous with "behaviorism" for many.

For studies on behavior ethograms are used.

Outside psychology

Behavior as used in computer science is an anthropomorphic construct that assigns “life” to the activities carried out by a computer, computer application, or computer code in response to stimuli, such as user input. Also, "a behavior" is a reusable block of computer code or script that, when applied to an object (computer science), especially a graphical one, causes it to respond to user input in meaningful patterns or to operate independently, as if alive.

In environmental modeling and especially in hydrology, a behavioral model means a model that is acceptably consistent with observed natural processes, i.e. that simulates well, for example, observed river discharge. It is a key concept of the so-called Generalized Likelihood Uncertainty Estimation (GLUE) methodology to quantify how uncertain environmental predictions are.