DOS may refer to any of the following:
1. Short for disk operating system, DOS is an acronym often used to describe MS-DOS and the Windows command line. More generally, it may be used to refer to any operating system that runs from a disk drive. Today, all operating systems can be considered disk operating systems.
2. Short for denial of service, a DoS attack is attacking a networked computer by sending it an abnormally high number of requests, causing its network to slow down or fail. Since a single individual cannot generate enough traffic for a DoS attack, these attacks are usually run from multiple computers infected by worms or zombie computers for a DDoS.