A Gettier scenario is a philosophical problem that challenges the traditional definition of knowledge as justified true belief. In these scenarios, a person has a belief that is true and justified, yet intuitively, they do not seem to possess knowledge. This raises questions about the nature of knowledge itself. The term comes from a 1963 paper by philosopher Edmund Gettier, who presented examples where individuals have beliefs that meet the criteria for knowledge but are based on faulty reasoning or luck. These examples highlight the complexities of defining knowledge beyond mere belief and justification.