Denial of Service: An attack on a network or service that aims to overwhelm its infrastructure and make it unavailable to its intended users. This can be achieved through various means, such as flooding the target with superfluous requests to overload the system or exploiting vulnerabilities that cause a crash. Common forms include Distributed Denial of Service (DDoS) attacks where multiple compromised systems are used to launch the attack, smurf attacks, and ping of death attacks.