DOS: Disk Operating System: DOS refers to an early computer operating system that primarily used command-line interfaces and was widely used in the 1980s and early 1990s before graphical user interfaces (GUIs) became more prevalent. Department of State: In a government and diplomatic context, DOS can refer to the Department of State, the executive department of the U.S. federal government responsible for international relations, diplomacy, and foreign policy. Denial of Service: In the realm of cybersecurity and hacking, DOS represents a type of cyber attack aimed at making a computer system or network unavailable to its intended users by overwhelming it with a flood of malicious traffic or resource exhaustion.