Quantum advantage is the point at which a quantum system solves a specific task better than the best known classical methods. The task may be narrow, such as simulation, optimization, or sampling, and the advantage may be measured in speed, cost, accuracy, or energy use. It does not mean quantum computers are universally faster; it means they win on a defined benchmark.
In cyber security, the term matters because a proven advantage on cryptography-relevant tasks could change the risk profile for encryption, authentication, and key exchange. Attackers may look for future gains against public-key algorithms, while defenders use the same concept to test post-quantum cryptography, validate benchmarks, and plan migration. A claimed advantage must be checked against classical baselines and reproducible conditions, or it may be only a lab result rather than a real operational threat or defense.