Denial of Service, often shortened to DoS, is a condition where a system, application, or network service becomes unavailable or unstable for legitimate users. The disruption may come from excessive traffic, malformed requests, resource exhaustion, crashes, or logic flaws that leave a service unable to respond normally.
In cyber security, DoS matters because availability is a core security goal: even without stealing data, an attacker can stop logins, block admin tools, interrupt hosting, or degrade critical business services. Defenders look for rate limiting, input validation, capacity limits, timeouts, and isolation of sensitive management interfaces. In real attacks, DoS is often used to extort, distract from other intrusion attempts, or create operational pressure while other defenses are busy.