Any behavior directed toward another individual that is carried out with the proximate (immediate) intent to cause harm. In addition, the perpetrator must believe that the behavior will harm the target and that the target is motivated to avoid the behavior.
Bushman, B. J., & Anderson, C. A. (2001). Is It Time to Pull the Plug on the Hostile Versus Instrumental Aggression Dichotomy?. Psychological Review, 108(273-279). doi: 10.1037/0033-295X.108.1.273.