The aim of the present research was to examine adolescents' definitions of bullying in a nationally representative sample of adolescents in Ireland. Definitions of bullying were examined according to age, gender, and bullying experiences. A sample of 4358 adolescents aged 12-19 years (M = 14.99 years, SD = 1.63) provided their definitions of bullying as part of the My World Survey-Second Level. The definitions were explored using content analysis. Adolescents differed in terms of their definition of bullying, with younger students frequently describing the nature of bullying as mean, while older students displayed a heightened awareness of the feelings associated with being a victim of bullying. Older females and those who had experienced bullying were more likely to discuss the emotions associated with bullying compared to males and those who had not been bullied. Adolescent definitions of bullying were not in line with widely accepted researcher definitions. Recommendations are made for researchers and those designing anti-bullying interventions and educational programmes.