Bullying is the use of force, coercion, hurtful teasing or threat, to abuse, aggressively dominate or intimidate. The behavior is often repeated and habitual. One essential prerequisite is the perception (by the bully or by others) of an imbalance of physical or social power. This imbalance distinguishes bullying from conflict. Bullying is a subcategory of aggressive behavior characterized by hostile intent, imbalance of power and repetition over a period of time. Bullying is the activity of repeated, aggressive behavior intended to hurt another individual, physically, mentally or emotionally.

Bullying can be done individually or by a group, called mobbing, in which the bully may have one or more followers who are willing to assist the primary bully or who reinforce the bully by providing positive feedback such as laughing. Bullying in school and the workplace is also referred to as "peer abuse". Robert W. Fuller has analyzed bullying in the context of rankism. The Swedish-Norwegian researcher Dan Olweus says bullying occurs when a person is "exposed, repeatedly and over time, to negative actions on the part of one or more other persons", and that negative actions occur "when a person intentionally inflicts injury or discomfort upon another person, through physical contact, through words or in other ways". Individual bullying is usually characterized by a person behaving in a certain way to gain power over another person.

A bullying culture can develop in any context in which humans interact with each other. This may include school, family, the workplace, the home, and neighborhoods. The main platform for bullying in contemporary culture is on social media websites. In a 2012 study of male adolescent American football players, "the strongest predictor [of bullying] was the perception of whether the most influential male in a player's life would approve of the bullying behavior." A study by The Lancet Child & Adolescent Health in 2019 showed a relationship between social media use by girls and an increase in their exposure to bullying.

Bullying may be defined in many different ways. In the United Kingdom, there is no legal definition of bullying, while some states in the United States have laws against it. Bullying is divided into four basic types of abuse – psychological (sometimes called emotional or relational), verbal, physical, and cyber.

Behaviors used to assert such domination may include physical assault or coercion, verbal harassment, or threat, and such acts may be directed repeatedly toward particular targets. Rationalizations of such behavior sometimes include differences of social class, race, religion, gender, sexual orientation, appearance, behavior, body language, personality, reputation, lineage, strength, size, or ability.