DOS is a group of similar operating systems for the IBM-compatible personal computer, including MS-DOS , PC-DOS , DR-DOS and FreeDOS
DOS/360 is an operating system from IBM for the System/360
DOS is an acronym for Disk Operating System which is an Operating System or operating system component that deals with high-level disk-IO (such as file-systems). Some examples include:
A Denial-of-service Attack is an attack in which no access to the system(s) is gained, but rather a loss of service is incurred, typically the loss of Network connectivity and services