Nonmonotonic reasoning is a type of logical reasoning where the introduction of new information can change previous conclusions. Unlike traditional logic, where once something is proven true it remains true, nonmonotonic reasoning allows for flexibility. This is useful in real-world situations where knowledge is incomplete or evolving. For example, if we know that birds typically fly, we might conclude that a sparrow can fly. However, if we learn that this particular sparrow is injured, we may revise our conclusion. This adaptability makes nonmonotonic reasoning important in fields like artificial intelligence and knowledge representation.